While there is no strong "role" for classes in Diablo 3, you can't deny that each of them is especially good at something. The barbarian can receive a lot of damage (tank), the demon hunter can perform long range attacks (archer) and the witch doctor summons lots of minions (necro).
Wizards and monks purposes are less obvious to me. I thought that the wizard's main strength would be the ability to deal lots of damages (either with area damages spells, or simply with high damages spells). I've been playing Diablo 3 with two friends from the beginning, and their monk and demon hunter seem to kill mobs as fast as I do. Moreover, they have a far better vitality.
We have the same kind of stuff and the same level. I maxed my intelligence with the gems system; also I tried several sets of spells and runes. Am I missing something ?