Just watched a game in which Hawks played the Heats on 2 January 2012: Hawks won 100 - 92.
Size: His length and size definitely matched that of LeBron's.
Speed/Quickness: This is where he'd struggle. He doesn't seem to sprint - he seems to just 'glide' along at a leisurely pace. May be P3 could work with him in the off season on that aspect? I think if you have a race, he'd probably be neck & neck with Millsap.
Rebounding: He's very good at boxing out and grabbing rebounds both offensively and defensively. Uses his lower body very well. His size and that lower center of gravity really helped him. I think we'll really feel the difference his length is going to have on our team rebounding total.
Transitional defence: As you'd expect, very slow getting back in transition. Guilty of ball watching at times.
Half court defence: Switches well on PnR - he easily slides in to guard/box out Bosh, again because of his size. He's not good at fighting through a solid screen though (may be due to his slowness?) and can fall over running into screens. He'll also have a hard time tracking down or staying with a quick SF on the move. He lost LeBron a couple of times which lead to easy alley hoops. I think he'll struggle with the likes of Durant who's always on the move and running off screens.
In the end, McGrady had a good night and finished the game instead of Williams.
He's not perfect by any means. I think KOC must think that we can somehow improve his foot speed & intensity in making this trade. Also please remember this is based on only 1 game.