Someone questioned his ceiling. That's odd... don't we question the ceiling of dudes that suck but are super athletic?
Shooting is the big area I see him improving in if does at all. Athletic ability you either have it or you don't ( he doesn't have it) the improvement there is finite. He is already skilled coming in the league. He was able to contribute immediately. His potential ceiling is less than some guys but he was more ready from day one.
He'll also likely improve in finishing around the rim, passing and defense. He's got a fairly high ceiling, especially if you compare it to realistic expectations for other players, and not the "Gerald Green's an exceptional athlete so his ceiling is league MVP" nonsensical ceiling.
Your last sentence captures my point - maybe better than I was able to express. The only issues with Sully, as another pointed out, is his back. If someone has a solid understanding of back injuries similar to his, I'd be interested to know what we might expect of him. I guess I jumped the gun by presuming he'll be okay (and I'm not concerned with weight issues, as others said.. simply b/c he seems locked in, motivated.. feel pretty confident he'll do what it takes to be successful).
I'd just place my money on him over an uber-athletic dude with lots of upside and little/nothing to show for it yet. He is a very good basketball player, his size has not proven to be a setback (he's succeeding at center!), and I can only see him becoming better and better as he learns the game. He's both talented and cerebral, motivated, and tough. I'd take that all day.