The thing that I question with Billy is his lateral quickness. He gave up baseline or got beat by his man on drives quite a few times in the preseason. The team defense of the Celtics will bail him out of these kinds of mistakes fairly often, but I'd rather they didn't have to. Other than that, he seems to have quick hands, and that's not really something you can teach.
The kid can score, though. The biggest play of his (other than the dunk) that stuck out to me was when he faked right and broke his man's ankles, took two steps left and knocked down a 17-footer. It wasn't the result or the play itself, so much as it just looked like he'd been doing it his whole life. A three-point shot would help him, but from what I've seen he's already got a decent midrange game - and of anybody in the NBA, Paul Pierce probably represents the kind of package that Walker could bring better than anyone, so it's good to see Walker with the opportunity to learn from somebody with a very similar game.