How do I pick? Which ever I think will give me the most enjoyment...

Assuming we are talking PvE here

1. Check what your raid/guild need.
2. Failing that check what your server is often looking for for pugs
3. Failing that role a hybrid (a full on hybrid: one that can heal, dps and tank (paladin or my personal favorite: druid)), and decide later.
4. Failing that look for an active community that you feel that you could get involved in such as blog-o-sphere, Elitest Jerks, maybe even here (watch out for the paladins, hunters, and most of all druids though as it's very hard to shut them up (as I hope i'm proving))
5. Failing that pick randomly and see where the wind takes you (I always find that fun and hence why I have an alt army at various points of the leveling process on alot of different servers)

If none of the above apply then I think you should ask your self the simple question: do I really want to roll a new character?

I had alt-itis through all of BC, which resulted in me now having 4 70s, a 76, and an 80. A few anecdotal lessons from all that:

Decide what is most important for your enjoyment of the game: class mechanics, role, or something else? Then choose your class based on that.

For example, my first two mains were a warlock and hunter just b/c I liked the extra complexity of controlling a pet. Eventually I realized I hated mana, so I tried a warrior. Hated rage too, so tried a rogue and found I liked the Energy mechanic the best.

Then DK's came out and I tested one, really liked the combination of Runes and Runic Power, the diverse talent trees (travel speed in UH, self-healing soloability of Blood, snap AoE of Frost, etc) and leveled one to 80. At 80 I decided I should learn how to tank, and found I liked that roll better than anything I'd done so far in the game.

Now that my DK tank is just working on weekly ICC raids with not much else to do, I've geared and spec'd my 40 war, 35 pally, and 76 druid for tanking, and I tank LFG randoms for fun during downtime on my main. I find I no longer dislike mana or rage, as long as I'm tanking. Funny how finding the right role diminished class mechanics as a criteria for my enjoyment of the game.

The point of all that is, there are several criteria for choosing a class, and you have to figure out which criteria is most pertinent to your own enjoyment of the game, and choose based on that. However, that criteria may not always be clear until you've explored and experimented a bit.

Depending on what your situation is you would pick whichever role helps the best for progessive and performance.

Say your guild needs another healer for a fight like Deathbringer, you may want to go heals. They may need another person with a tank offspec for certain fights so you may want to go tank. They may need any number of things which each class fulfills in a different way and it can usually come down to preference on which you enjoy the most. Dual-Spec is really a great tool for this because you can have your chacter go any combination of specs depending on what suits you best.

• Tank: Paladin, Death Knight, Druid, Warrior
• Tank Healer: Priest, Shaman, Paladin
• Raid Healer: Priest, Shaman, Druid
• Pure DPS: Rogue (Melee), Mage, Hunter, Warlock (all ranged)
• Utility DPS: Nearly every class has some sort of utility to go with them, just some more than others. Rogue can detect traps in ICC, mages can sheep, DK's chains of ice, pally DI, druid BRez, shammy hero, etc etc

To go into the specifics of DPS

• Melee: Warrior, Death Knight, Paladin, Rogue, Enh Shaman, Feral Druid
• Ranged Physical: The only class in this category is Hunters
• Ranged Caster: Mage, Warlock, Priest, Ele Shaman, Balance Druid