I used to go to UNC til this year and saw Hansborough play a lot in person. I think his ceiling is an Antawn Jamison type with hustle - he'll need to continue improving his jumper to get there, but Jamison didn't have much of a jumper in college either.
At the very least he'll be an energy player with a knack for fighting his way to rebounds and a great ability to put the ball in the basket from every possible angle, even after contact. Picture Leon Powe with a softer shooting touch. He's also remarkably injury free for all the contact he takes (this year's shin problem is his first real injury, and it wasn't incurred in a game). I think he's practically guaranteed to be a contributor no matter where he winds up.
The biggest negative, other than questionable athleticism, is that by all accounts he is dumb as a rock, so his ability to learn a pro scheme may be hindered. But he'd hardly be the only intellectually limited player in the NBA, so that may not be a factor.