This is more gushing than it is analysis, but I thought I'd share.
I enjoy the hell out of Diablo 3's characters, pun intended. The main characters, that is. The Barbarian, Monk, Wizard, Demon Hunter and Witch Doctor all have quite a bit of personality. And what I find most interesting about them is that they embody the world they are from: They are between heaven and hell.
Each character embarks on the quest to eventually kill Diablo, all for different reasons. The Barbarian for a worthy challenge, the Wizard to fulfill his or her destiny, the Demon Hunter for revenge, and the Monk and the Witch Doctor answer to a higher power. They are not doing so for altruistic reasons. They are called to do so by their own will and choice. And all of them are not good or evil. Even the Monk, who is the most divine character, flat out tells the mayor of New Tristram that he is a coward and should die. His conversations with the Templar note that he is typically sent to hunt down criminals. There are not nice people, and I think that's fantastic.
It is quite easy to get into the head of one of these characters (except the Witch Doctor, but anyone who spends their free time collecting jars of spiders is one train ticket away from crazy town) and have very clear motivations. They fight beings of pure evil and ally themselves with beings of pure good, but they distance themselves from both. They fight evil because it is destroying the world, and hang out with good because Tyrael is trying to save their asses. Their motivations are fairly selfish, and I'm sure if the demons weren't threatening the world none of these characters would care about them.
They represent humanity. They are selfish and concerned with their own world and well-being, and it works. Diablo 3 isn't about saving the world to fulfill some higher purpose. It's about killing demons because F#$% demons.