Can I play old World of Warcraft expansions content on the latest expansions?

I know that I need all the previous expansions to play the latest one. So since I want to buy the game and with all of the expansions at once, can I play all the expansions contents on the latest expansion?

Including the original features of world of Warcraft. In other words, can I play everything that is in the original:

• World of Warcraft
• Burning Crusade
• Wrath of the Lich King
• Cataclysm

In the mists of Pandaria?

Or do I have to reach the maximum level on the original, then play Burning Crusade to level up more and its content and once I'm finished there move on to next one, etc. I don't want to waste any money if I get the wrong thing.

Much of the original content (quests, dungeons, raids, etc.) from each of those old expansions is still available and is still playable. Some exceptions exist; for example, the level 60 version of Naxxramas from version 1.11 is no longer accessible within the game, since it was replaced with the Northrend version in Wrath of the Lich King. Many old quests, NPCs, and zones were removed or redesigned completely when Azeroth was reshaped for Cataclysm. Some things have endured unchanged since their implementation, however (such as most of the Outland content) and that content is mostly still perfectly accessible.

However, wide system-level changes have occurred to the game many times over in the intervening time, and those changes apply to all players, regardless of their level or what expansions they own. For example, the talent system has been redesigned several times over the lifespan of the game, and all characters are subject to the latest version of that system; it's impossible to play the game today using the talent system from an earlier period in time. Similarly, more recent feature additions such as pet battles, achievements, and the dungeon finder are accessible to all characters.

As far as "what content can I play with which expansions," you need to own every expansion in order to level up to the maximum level cap. It is not possible to play through all of the previous content all the way up to the current level cap simply by buying Mists of Pandaria and skipping all previous expansions. (Since the base game now automatically includes all expansions up through Mists of Pandaria, though, this point is largely moot.)

Updated for Shadowlands (patch 9.0) If you buy the base game...

• You get the basic races and can unlock the allied races.
• You get all classes, including deathknight, monk and demonhunter.

You get access to all content that is in the game prior to shadowlands to level from 10-50. This includes Burning Crusade (Outlands), Wrath of the Lich King (Northrend), Cataclysm (Eastern Kingdoms and Kalimdor), Mists of Pandaria (Pandaria), Warlords of Draenor (Draenor), Legion (The Broken Isles) and Battle for Azeroth (Kul Tiras and Zandalar).

You get access to a separate installer/server for wow Classic content (Eastern Kingdoms and Kalimdor). To play that content, make a character specifically for that server.

Shadowlands is the current expansion available for purchase. If you buy it then you get level 50-60 Shadowlands content.

You don't "install" expansions; everyone uses the same game client and the same servers regardless of what expansions you have purchased.

Instead, the expansions only unlock the higher level content associated with them.