What are some pros and cons of the different races?
Solution 1:
Since this is a very large topic I'm going to try and break it down in to a series of sections
- Macro Mechanic (one of the new Features in SC2)
- Unit production and Tech tree
- Expanding
- Harassment
- Defenses
- Unit compositions - ability to adapt
- Upgrades
- Air
I broke up my answer into a series of race specific answers. You can find them here:
- Protoss
- Zerg
- Terran
Solution 2:
Protoss
"For destruction, Ice is also great; and would suffice."
The changes made to Protoss between Starcraft Broodwars and Starcraft 2 have lead to a race with a completely different feel. Based around the Warpgate Protoss forces easily resupply in mid combat with some of the toughest units in the game. Known for valuing quality over quantity the Protoss force tends to be small but powerful.
Macro Mechanic
The Protoss macro mechanic is the Chrono Boost. This ability allows any building to increase the rate at which it is performing by 50%. This includes research and unit production. As a result Protoss can quickly upgrade and progress up the tech tree or produce strong forces on the fly. This is perhaps the hardest macro mechanic to use, not because of the tension (deciding what to use it on), but remembering to use it. Especially in late games you will see Protoss Nexus with unused Chronoboost.
Unit Production
While back in Starcraft 1 Protoss were unique in that their buildings built themselves without the help of a worker (once started), this has further been augmented in Starcraft 2 to allow the same production of Units! Warpgates allow for the production of Gateway units (Zealot, Stalker, Sentry, High Templar, Dark Templar) anywhere on the map where the Protoss has power (anywhere they could build a building). This is further bolstered by the Warp Prism which allows power to be distributed anywhere it can move to (which as a flying unit is anywhere). The result is unparalleled map control. The ability to field a force where ever, whenever and reinforce it. This also lead to Protoss builds being heavily Gateway unit dependent. The Protoss techtree then usually looks like a thick trunk with 2 or 3 major branches. This trunk is the large bulk of gateway units, which are later supplemented by units from the Robotics Factory or Starport (or more Gateways). This also leads to a "Sim City" style of play, where Protoss players seek to produce a high number of Gateways (to turn into Warpgates). Its also worth mentioning that Warpgates decrease build times.
Expanding
Protoss expansion is fairly easy because new Nexus can be warped in and probe production can be boosted by Chrono Boost. Additionally, the ever flexible Photon cannon leads to cheep but effective defense. Finally, the presence of power around new expansions means that a Protoss player can respond to Harassment almost immediately. Without having to wait on the bulk of his forces.
Harassment
Protoss actually have a verity of tools at their disposal for harassment. The Charge upgrade for Zealots, in addition to providing the Charge spell, is also a movement speed upgrade. This allows your Zealots to work as an effective harassment force. Stalkers are fast to begin with, but also benefit from Blink as it allows them to bypass cliffs (in a manner similar to Reapers). Both of these are excellent harassing units right out of the Gateway, but Protoss' real harassment power comes from the Warpgate/Warp Prism combo. Warp Prism functionality is fairly straight forward: Move to an area, activate Prism to produce power, warp in units. What people frequently forget is that units can be loaded into the Warp Prism as it is a transport. This means you can drop an Immortal (or other ground unit) in the back of someone's base and then Deploy the Prism for reinforcements. Alternatively you can Deploy and warp in units and then load them up when the Opponent brings back troops to defend. The Warp Prism is a very versatile and often underused unit. Additionally, Phoenix work very effective as a harassment unit. Their ability to shoot and move provides air dominance over a number of other air units, but it is the Graviton Beam that really makes the Phoenix shine. This allows a Phoenix to lift workers so that other Phoenix can shoot them (remember workers are light armored units). Finally, the Voidray is often used as a harassment unit due to its ability to quickly destroy buildings. 2 or 3 Voidrays will make short work of a Command Center/Nexus/Hatchery often times before reinforcements can arrive. Don't send one in alone though, as they are expensive and take too long on their own.
Defenses
While the Photon Cannon is known for being reliable (Attacks both ground and air and is a detector for stealth units). Protoss defense usually trades on their ability to control the map. Fast units like Stalkers (which can teleport short distances) plus the ability to make impassable terrain (Sentry), added to great anti air (Pheonix which can move and shoot) combine with Warpgate technology to produce an army that can control when and where battles take place. This ability becomes the greatest defense the Protoss have, choosing their ground. Just ask the defenders at Red Cliff or Thermopylae, you control the terrain you control the battle.
Unit Composition
As mentioned before Protoss tend to rely on heavy gateway unit compositions with a few others thrown in. Because a given Protoss based can support 3 Gateways plus an additional building of constant production, Protoss strategies are often described as "3 Gate into Robo" or "4 Gate" or "3 Gate plus Starport". In the case of "4 Gate" this often refers to producing some sort of templar out as well. The Protoss ability to adapt is often based around what this fourth building is. This leads many to believe Protoss is inflexible in early to mid game. While this maybe true, they make up for it by having a very powerful core unit group which lasts from early to late game.
Upgrades
Protoss also have only 5 weapon/defensive upgrades. However, since Protoss have two different defenses (armor and shields) this often leads to just having weapons upgraded. While this can be frustrating, they make up for it by being able to Chrono Boost upgrades in progress, leading much faster upgrades. There are a few key upgrades outside of basic weapon/defense, such as warp gate or thermal lance, but their low number makes them very accessible.
Air
Protoss Air is usually considered an after thought by many players. This is a mistake. While Protoss may lack the massive airpower Zerg has, they are not to be underestimated. For starters Protoss have the strongest Air to Air unit, the Pheonix. In additon, to being able to move while it shoots, the Pheonix is able to lift up ground units, effectively taking them out of combat for the duration, and making them target-able by other Pheonix. This strategy is very effective against high powered units like the Terran Siege Tank. Added to this, Protoss have the very scary Void Ray. The Void Ray is a unique unit in that the longer it attacks a target the more damage it does. This ability, combined with its bonus versus armored, has lead to players using the Void Ray to snipe buildings. This tactic, often employed as a rush, is most effective against Zerg who have late anti-air.
Solution 3:
Terran
Boasting the highest number of units, as well as some incredibly cool niche units Terran is often considered the most powerful or the most cheesy race, depending on who you talk to. While actual statistics tend to show Terran to be more balanced than not, this massive unit diversity often leads players to think otherwise.
Macro Mechanic
The Terran are unique in that they have two different macro mechanics. First they have the Orbital Command center which allows the production of MULEs which harvest minerals at 3x the rate of normal workers and allow normal workers to harvest the same Crystal at the same time (a net result of an increase of 3 workers even when fully saturated). Its worth noting that the limited life span of the MULE means that it will harvest 240-270 minerals over the span of its life. While the Orbital Command increases economic production, the Terran have a different mechanic to increase unit production, the Reactor. Reactor allows for double production of units from an attached unit building structure. Since these two mechanics do not have the tension between the two that is present in the Zerg or Protoss mechanics, both mechanics are limited in important ways. While a Reactor allows for double production, it is of only certain units (Marines, Hellions, Vikings, Medivac). The other units require an attached techlab for production instead. This limits production of other units or those units. Fortunately the Terran build's ability to fly allows for quick switching between addons. The Orbital Command is balanced by being forced to spend energy on either a MULE or Scans or upgrading Supplies. Usually players will tell you that MULEs are always the best choice as the opportunity cost is at least 240 minerals. Additionally, a Terran player must choose between Orbital Command and Planetary Fortress (an excellent aoe defensive structure). In comparison to the large number of decisions a Terran player must make about their macro mechanic, it is relatively easy to use. Excess energy for MULEs can be used later with little cost, buildings with addons don't require additional attention to use.
Unit Production and Tech Tree
While Terran Units may build similar to Protoss (from structures), the units are spread out over a larger number of structures. This often leads to Terran players going down one of three paths. Heavy Barracks units or heavy Factory units or an even mix between the two and Air units. While this may seem like a Heavy toll, the large unit diversity means that all three of these production strategies are valid. The resulting tech tree is also very wide which can either lead to amazing flexibility or an over specialized build.
Expanding
As you'll see more in section 5, Terran has a very heavy defensive arsenal. As a result many players will make few expansions. However, this practice is not characteristic of the race as a whole. The ability to fly Command Centers, combined with Planetary Fortress allow for safe expanding for even fortified Terran. Command Centers can safely be constructed behind allied lines and moved into position. Once landed Planetary Fortress provides for relative safety from harassment until more effective fortifications can be constructed. It is worth noting that Planetary Fortress cannot be lifted off, and should only be constructed after the Command Center has landed, and even then sparingly.
Harassment
Terran harassment is characterized by very niche units. Terran units can be divided into three categories: Harassing, Defensive, or Bulk. In the first category is made up of Hellions, Reapers, and Banshees. Each of these units is used in a different fashion but almost all are most effective against workers. Reapers trade on their ability to jump over cliffs this ability allows them to surpass any natural defense. While they are effective because of this, their low hp makes them fragile (a recurrent theme). Hellions, by comparison trade on their high speed and AOE damage, as well as incredible bonus vs light units (which workers are). As a result they are most effective behind mineral lines or attacking fleeing workers (which naturally form lines same as Hellion AOE). Finally, Banshees are fast air units with very high damage vs ground. They further benefit from a stealth upgrade which makes them invisible to players without detectors. All these units are highly offensive, but weak when pinned down, making the poor for pushes and defense.
Defenses
Like harassment, Terran have very specialized units for defense. No unit epitomizes this more than the Siege Tank. Back from Starcraft 1, the Siege Tank has been upgraded in two important ways: - Siege Tanks AOE damage is now centered on its target (and larger) meaning it will strike more of an incoming force - Siege Tanks will coordinate to reduce overkill (multiple tanks attacking the same target when fewer would suffice.
These two changes result in such an effective defensive unit that a "moving defense" has become a viable strategy. This is accomplished by moving Siege Tanks forward in waves (similar to Starcraft 1). To further bolstered by Turrets and Planetary Fortress, both of which have good upgrades from the Engineering Bay, making them very effective. Finally the Bunkers from Starcraft 1 return with the ability to sell them off when they no longer become useful.
Unit Compositions
Terran unit compositions are based around unit producing structures (section 2). These are further complimented by their upgrades (section 7). The result is unit compositions that tend to be made out of Marine/Marauders or Tank/Thor or Marine/Tank/Viking; the last of which chooses heavy diversity over sharing upgrades or production structures. This often leads to tough and slow pushes, with more aggressive units mixed for harassment and winning battles. This leads Terran to really excel at mid/late game.
Upgrades
More than any other race Terran require their upgrades to function. Every unit has its own upgrades (except Viking/Medivac) all of which are useful and many necessary (Concussive Shell, Siege Mode, Stimpac, etc). The result is a force which often times is very inflexible and unable to adjust to rapid changes in strategy. To make matters worse Weapon/Defensive upgrades are different for each unit producing structure (6 different upgrades).
Air
While not the best, Terran air is no slouch. Vikings provide very long range and excel at killing Air units which are effective against ground. Banshees provide great harass (section 4). Ravens are an ever useful caster; so useful that many strategies become unstoppable with the addition of Ravens. Medivacs are often used to bolster Marine/Marauder forces making them not only tougher but more mobile. Finally Battle Cruisers have recently been shown to be extremely effective in even pro matches.
Solution 4:
Zerg
One of my all time favorite races. I think the thing I enjoy the most about the Zerg is the idea of them. Nothing to do with actual play style or units, but rather the idea of a ravenous horde constantly adapting. Unkillable.
Macro Mechanic
The macro mechanic for the Zerg in SC2 is based around the queen. Traditional Zerg production is based on larva (as discussed in section 2). Larva are in turn produced from the Hatchery structure. To increase larval production in Starcraft and Broodwars meant building additional Hatcheries. This is revolutionized in Starcraft 2 by the addition of the queen. Unlike traditional Zerg units, the queen is build directly from the Hatchery (similar to other races unit production) and has the ability to infuse the hatchery with additional Larva. The result is that Zerg play becomes largely focused around unit production. As one of these units is the Drone, this often leads to heavy macro style play as zerg economy becomes exponential based on the production rate of drones. While this macro heavy style is facilitated it is not the most common style of zerg play. The other alternative is to devote this extra larva production to units. This leads to massive forces produced very quickly. Finally the queen has two other abilities to add tension to this mechanic (which will be discussed in sections 4 and 6). Because larva injection is a blocking operation (only 1 per hive at a time), this macro mechanic is often criticized as being the hardest to use (though I might say the Protoss one is on par).
Unit Production and Tech Tree
Zerg unit production has not changed since Broodwar. Units are still morphed from larva. Larva are still produced from hatcheries (though faster with the queen). There are still units produced from other units, most notably banelings and broodlords. The Zerg tech tree is far more linear than its Protoss and Terran equivilents. It can easily be broken up into "Tiers" based around upgrading the Hatchery. The result is that high tech takes an extra delay as this upgrade must be built and it is more vulnerable as player rarely upgrade more than one Hatchery (there is no additional benefit). Many players will say that the Zerg has the most flexible tech tree as all units are built from a single structure. While this is true, the flexibility is countered by each building adding only 1 new unit (with the exception of the Spire) and costing a drone which further reduced resource production. The result is that late game change in composition is easy, where as early game it is nearly impossible.
Expanding
At 350 Minerals (300+50 to replace the drone) the Zerg have the cheapest resource gathering building. The result is that expanding with Zerg becomes very easy. In fact, it has become so easy that it is vital in many strategies. Since Zerg units hit their strength in the midgame (section 6) which tend to be high in gas. It is often a popular move for a Zerg to take a new expansion and immediately saturate the gas (build two gas production buildings and assign 3 workers to each) before the minerals. The result is an influx of an otherwise rare resource (most comparisons rate gas as being 2.5 minerals in value). It is worth noting that many macro style Zerg strategies (section 1) require very early expansions (sometimes called FE for fast expansion) for a massive boost to the economy (doubling larval production as well). Its worth noting that most people believe that Zerg must control one more expansion than their opponent to be effective. Though the advent of the queen has lead to a number of 1 base strategies (no expansion), expanding is still very common play for Zerg.
Harassment
As others have mentioned Zerg are an extremely mobile force. This mobility is boosted by a number of upgrades (Zergling, Baneling, Roach, Overlord), the addition of easy creep production (Creep Tumors and Overlords), and the advent of the Nydus Network. The Nydus Canal was a building in Starcraft 1 which allowed Zerg to connect any two points on the map (provided they had creep). In Starcraft 2 ALL Nydus Canals are connected to each other (leading to the connection of N points on the map). Additionally, Nydus Canals no longer require creep to be built on and even produce creep. The result is even slow units (Hydralisks) can become extremely mobile and Zerg are able to field large armies in rapidly different sections of the map. To complement this, creep now acts as a speed boost to Zerg. Any Zerg units on creep naturally move faster. Additionally, some Zerg units are penalized for being off creep (Queens). This is further bolstered by the fact that Overlords can produce creep starting at Tier 2 (the upgrade of the Hatchery to the Lair). Additionally, Queens can produce what are called Creep Tumors. These Tumors must be built on creep but readily spread creep. While this may seem lackluster, the tumors are also cloaked and able to produce more tumors. The result is that Zerg often construct creep highways which are expensive for opponents to destroy. Zerg has some of the most mobile units in the game. All Tier 1/1.5 units can receive speed upgrades (Zergling, Baneling, Roach) making them the fastest land units in the game (Zerglings). The Mutalisk (by comparison) is one of the fastest Air units in the game. Its high mobility has lead to the technique of Muta-Harass, where Mutalisks attack workers in Guerrilla style fashion and devastate opposing economies. Mutalisks are so effective at this that both Terran and Protoss have specific units for countering them (Section 8).
Defenses
Zerg are often criticized as having the weakest defense in the game relying more heavily on units. This is an exaggeration. Zerg receive an early unit (the Queen) which has good air and ground defense as well as having a powerful heal for both units and defensive structures. Additionally, Zerg defensive structures can be uprooted and reused. The result is a very cost effective defense force. While in Starcraft 1 the production of defensive structures was often seen as admission of a mistake, in Starcraft 2 professional players feel free to build them liberally, using them to both augment forces as well as control the map.
Unit Composition
Zerg production rates mean that compositions are often dictated by resource gathering more than anything else. The result is compositions with both Gas heavy elements and Mineral "dumps." This usually lead to compositions like: Hydra/Roach, Ling/Muta, Infestor/ling/bane, bane/ling/ultra and so on. When constructing a force a Zerg player is often forced to think "Where do I want to spend my gas?" The answer to this question often dictates her strategy (Zerg get the feminine pronoun for Kerrigan). One of the major problems with Zerg unit compositions is the lack of any real anti-air until Tier 2. This is a result of the fact that the Queen is the only unit that can attack air before Tier 2 (Hydralisk/Muta/Corruptor at Tier 2). This may force players to tech up faster or build excess defensive structures to deal with early air harassment. While the initial Zerg units tend to be fierce, Zerg don't hit the strength of their force until Tier 2. The advent of high damage Hydralisks and effective harass of Mutalisks often make the mid game the strongest part of the Zerg offensive.
Upgrades
Zerg have a high number of unit specific upgrades. These usually consist of speed, burrowing, or increase of another ability. Additionally certain units can be upgraded into other units (Zerglings into Banelings, Corruptors into Broodlords). By comparison Zerg have the fewest basic damage upgrades (5 on par with Protoss). This usually leads forces to either be designed around Ranged Damage, Ground Damage or Air so as to share these effectively.
Air
Zerg air is the most powerful or least powerful depending on who you speak to. Zerg Mutalisks are one of the fastest air units, Zerg Corruptors are very good Air to Air units (better than vikings worse than phoenix), and Broodlords are an absolutely devastating unit in their own right providing unmatched air to ground. The Broodlords in particular are such a powerful unit. The Broodlord is designed around the traditional Gaurdian from Starcraft 1. A long range air unit that does high damage to ground units. To further bolster this, when a Broodlord attacks it produces a broodling. This is a weak ground attacking unit. While the broodling does additional damage, the most effective part is that enemy AI will target it over the Broodlord. Additionally, broodlings will physically block enemy anti air forces from attacking the Broodlord. Many professional players say that even if you took away the broodling's damage, the Broodlord would be just as effective.