I think each class will have their own specialty that'll create a sort of rock-paper-scissors system between the classes so there won't be "the one good one." It might just seem the Warlock is OP because, so far, it's the class we know the most about right now. Who knows, maybe the Hunters can go invisible or the Titans have some kind of temporary invincibility or insane shielding buff of some kind. Too early to say, sadly. Here's hoping more info gets doled out at GDC next month . . .