Dribbling is the biggest thing for him to work on. Getting past that first defender is critical.. he almost never took more than 1 dribble this season
Shooting is obviously big too... I actually suspect that he worked very hard on his jumper over the past off-season, based upon the percentage of his shots that dropped early in the year... but with his horrible form, there's just no way to be consistent. He has SO much movement in his lower body on his jumper. Fix the form and he has the work ethic to improve his percentage pretty significantly.
Then, obviously there's the strength. I love his leaping ability, but there were a couple scary moments when he was just knocked around going to the basket, and you could tell that he just didn't have the strength to play that way all the time the way a Vincent Grier did.
But, first things first: work on that handle. That'll change the way guys guard you and make it easier to hit the jumper.