Because NFL GMs put so much stock in combine results, if Hart's not a workout warrior (and I don't know if he is or is not) then he'll be a 3rd rounder tops. There's an injury risk factor, along with the heavy work-load (as it relates to his size) and I don't think the Michigan pedigree of NFL backs will help much (Unless the fuckin Bengals have 60 first and second round picks)... likewise, his mouth won't help either, particularly in this new "walk a straight line" NFL. That's not to say he's a troublemaker off the field, but I'd figure his yap raises some flags.
That said, the kid runs hard, seems to play whenever his body is able (though I still don't know why he was riding a bike against Appy State in the 3rd Q... I suspect it's coaching, though, and not injury since he ran fine when called on) and has shown a great deal of ball security. He seems to seldom fall on the first hit, and - contrary to the down side of a heavy workload - has shown he can A) take a pounding and B) can be a 30 carry back if needed.
He runs between the tackles, but if they're going to want a guy to get to the edge, he's not that guy. This factor may also hurt as it relates to size, in as much as taking a punishing from NFL LBs would scare me away from risking much money on the guy before he "proves" himself at the next level.