I think the biggest issue with Brown is his blitz pickup. I don't think that Peyton trusts him and for good reason. Yes, he led the NCAA in rushing at UCONN, but that was UCONN. I haven't seen anything at the pro level to indicate he's going to be a starting RB going forward. I think with what the Colts do with Addai being a free agent after this season will be telling.
The biggest issue is that he just can't stay healthy. I think the whole "he can't pick up the blitz" excuse is overused and possibly untrue. Brown has shown he is unreliable in one area and that is staying on the field. And I don't what you've been watching but I've seen enough to lead me to believe that Brown can be a solid, productive back if he is able to take the field consistently and wins the right to the opportunity. Hopefully the next few games will show us what direction he is going.
For the Colts sake, I hope Brown is a great RB. From what I've seen, he's the typical guy who plays well at a small school, but isn't that great at the pro level. He is supposed to be a smart kid, but has a had a very difficult time picking up the Colts offense. He has very good speed, but doesn't seem to read plays well, or hit the right hole most of the time. He's a terrible pass blocker, and I think that's more just not recognizing what's going on on the field than a lack of willingness. Perhaps with more exposure on the field, he'll learn. But right now I wouldn't pin my hopes on him.