Is a tank required for groups to be successful?

My friends and starting to play Diablo 3 and are deciding which class each of us wants to be. One thing factoring into our decision is if one of us will need to be a tank for us to be successful.

Will one of our players need to play the role of a tank, taking most of the hits for the team for us to succeed? Or is the game balanced to work with any combination of classes/roles?

I'd like to tell you that you can play with whatever you like - and for the most part, that's true. However, on higher difficulties, some group combinations can be extremely frustrating and benefit immensely from having a Monk or a Barbarian on the front lines who can soak hits.

While there's no concrete (or if there are, they're currently unknown) mechanics governing things like threat - monsters will smash just about anyone in melee with them, and will often sprint off to chase squishies if they want to - there's a definite inclination for most types of monsters to attack things directly in front of them. Barbarians and Monks, depending on build and items, can soak a considerable amount of damage up and even give their allies defensive boosts that other classes don't always have available to them. Without them, kiting becomes utterly mandatory in all scenarios against Champions and Elites in Hell and beyond, and certain ability combinations will end in suffering and sorrow.

Of course, in the higher difficulties, certain Champion/Elite abilities will tear through those heightened damage soaking abilities trivially anyway, and some monsters (Mallet Lords!) will smash them flat regardless of damage soaking - so awareness is still the most important trait a player can have, and you won't find that on gear.

TLDR: You can succeed with any combination of classes, but a diverse set of classes will cut down on your repair bills and your heartaches significantly. Choose carefully if you plan on going to Hell or Inferno.

From what I've seen of the game so far, I don't think you really need a designated tank, but you may wish to be somewhat careful about your ability/gear mixes on each character.

This kind of boils down to the fact that the "holy trinity" (tank/DPS/heal) really isn't present in this game - there's no healer, in particular, and each class has DPS-style skills and tank-style skills. Although every hero is going to prioritize their primary stat, and some stats are better for defense than others, there's a wide variety of gear and skills for tanking available to everyone. Wizards aren't limited to low-defense "cloth armor" or anything.

Most of the classes can be spec'ed to be more or less "tanky" - for instance, I played several hours as a Witch Doctor with a Wizard, and we both survived extended periods on the front lines. With life steal items on my Witch Doctor, I can hit hard and hit often, and my health will bounce back as my mana drains. Then I can cast a crowd-control spell with mana recovery, and go back to pounding away with the DPS again. The Wizard was using a "diamond skin" spell that boosts defense, and would teleport out of combat if things are getting a bit too hot.

I would suggest that you each play whatever class you feel suits you best, and at least at the lower difficulties you can choose your abilities how you wish. When you start to get where the game is getting harder, you'll want to make sure that everyone can survive for at least a little while in a melee, and ensure that you're all properly balanced for survival.