The first thing I want to see is that he can make it through the entire season healthy, that will make me start to believe that he wasn't another wasted 2nd round pick. After that I would be happy if he got 1200 yards and 8-12 TDs, which if healthy, I think he is more than talented enough to do.
I also want to see MBIII take another step forward this season and prove he is a capable backup/change of pace back. I think if both players show that this seasons and Thompson can come in and break off a couple long runs every now and then then our running game will be sewd up for several seasons. Anything less than that and we have to start evaluating our RB position for next years draft.