Diablo III won't have any old classes returning

Okay that's not entirely true since you've already got the Barbarian there but according to an interview that Gamespy links to, the reason the Barbarian class is still in the game is just because the class they were planning just seemed so similar to the Barbarian that they decided to keep the name. However, it was pointed out that even though Blizzard would not develop rehashes of old classes, the new ones would definitely find new fans and if they didn't there was the possibility of reintroducing the old classes in expansion packs.