Bashiok fielded a question about character collisions and physics, in the forums on Friday:
I noticed in the 2009 b-roll that the characters walk through each other. This was not the case in d2. I like the realism of solid characters, anyone agree?
Bashiok: For the co-op game allowing players to clip through each other just makes for a better and more enjoyable experience. You’re already dealing with enemies surrounding you and blocking movement, there’s no reason for your friends to do the same.
Of course clipping wouldn’t be able to work the same way in PvP situations, but we’re not talking about that yet.