I think I nailed why I thought Diablo 3 looked like an updated Dungeon Siege: It’s the jungle. The first level on Dungeon Siege is in a jungle and they look a little bit like each other. Other than that, I don’t think there are many similarities.
Also, one thing said in the presentation isn’t actually new: changing your attack with the mouse wheel. From what I got in the presentation, you can change your attack just using the mouse wheel or the tab key. On Diablo 2, you could do the same thing: you could assign a key to an attack (up to 10, IIRC, all defaultly binded to the function keys); using the mouse wheel would change the attack on one of the mouse buttons to the next one (I can’t really remember what would happen when you assign keys on different sides.) Anyway, I don’t think Blizzard would say something is new when it isn’t actually, so I’m guessing the systems doesn’t behave like in Diablo 2.
Still waiting for the release date. ;)