In Diablo 3 there are 5 classes:
2 melee: Barbarian and Monk
2 ranged: Demon Hunter and Wizard
1 pseudo-ranged: Witch Doctor
Why "pseudo"? Because to me the Witch Doctor doesn't seem like a pure ranged class. It doesn't fire projectiles directly at the enemy. Instead, it summons semi-intelligent minions, or fires semi-intelligent missiles (such as toads, spiders, zombies, bats, etc) in the general direction of the enemy, and these missiles sometimes home in on the enemy or explode in its vicinity. Well, you can always use Poison Dart but one skill still doesn't make it a proper ranged class.
I am very partial to ranged classes, which means that I won't pick the Witch Doctor from the very beginning. Unfortunately the Demon Hunter comes out as weak from what I can see in the beta. Not terribly weak but unexciting enough that I won't be playing it either, from the start.
This leaves the Wizard and boy am I hyped to play it. The Wizard has so many cool spells that I'm afraid I won't have enough space in my active slots to use them all. Some of the coolest I've seen are Electrocute, Arcane Orb and Disintegrate, not to mention Frost Nova.
The choice is more clear cut in regards to melee classes. I choose the Barbarian, of course, because it uses big honking weapons, while the Monk seems to only use his/her weapons for their stats (remains to be seen how things evolve later). I can't wait to use the Barbarian's Bash, Cleave and Leap Attack, at least in the early game.
Eventually I will try all classes, hopefully taking them to the max level. My first two, though, will definitely be the Barbarian and Wizard.