His man to man defense is actually pretty solid at this point, particularly in isolation situations. Where he has issues is off the ball he tends to overhelp/get caught ball watching and sags too far off his man, leaving them too open for jumpers. That's something that could pretty easily be fixed with some quality coaching. It didn't help that the Nets were pretty desperate for shot blocking and were probably ok with him sagging off his guy to help on the inside. The biggest difference you'll see is he's just incredibly active on this end now, effort definitely isn't a problem anymore, he always seems to be trying to disrupt things, even if it leads to some iffy fouls at times.
Offensively he's developed a real nice post up game and is very effective going to work from midrange. If he's starting outside the three point arc he still isn't very effective at creating his own shot (anything more than 2 dribbles becomes a turnover waiting to happen), but he tries to force less than he used to. He uses his athleticism to convert inside pretty effectively now as well, though he still has no idea how to draw a foul.
All in all, he's a guy who can put up some points quickly and relatively efficiently as long as he's not asked to handle the ball too much. Defensively I don't think he hurts you anymore, he may not be a huge plus, but on a good defensive team like the Celtics he'd be fine. I think he'd be a really, really good fit here though, so I'd love to see it happen. Chances are though he's looking for mini-midlevel type money, and if he doesn't get that he's likely to go back to the Nets for the minimum (he's incredibly grateful for the opportunity they gave him).