Q u o t e: The names have reflected how the 'good' guys aren't always knights in shinning armor- Barbarian, Necromancer, Assassin, These would have been the bad guys in another game.
I'm not sure about the good/bad, light/dark thing here.
Sanctuary is a world of gray, really its just mankind for its self. I don't see supporting either demons or the heavens making much sense in this case.
Septic, it's not about BEING a good guy or a bad guy, it's about picking sides. If the demons want to own the world, and you are good with that, then you pick the dude you like most. Maybe that is Mephisto 'cause of his big 'undead' thing. Maybe Diablo 'cause he rocks the house. If you are with the angels, then you can pick one of them. Tyrael, or one of the angel 'high council' members, whatever the hell their names are. Not like good or bad matter anymore, with this backwards goofy lore. Anyways, the point is that you, starting in act 5, have different sides. You fight for your side, and different quests.
Also would ROCK in PVP, as you could have some sweet team battles.
I am even thinking different 'epic' skills, where you get some bonus for picking a certain class. Would make for some sweet variability, even between classes.
Course you could still party with anybody, it's just that you have to do your class-specific quests in PVM.