It's sometimes difficult to judge Johnson's translation to the NBA because he's often playing out of position ... or doing things out of position. Without Hummel on the court, he's had to shoot a lot more jumpers this year. What you see there is that he's got a solid 18-footer for a 6-foot-10 guy. What you don't see is anything in the paint.
He'll have major problems establishing position in the NBA because he's just not strong enough. I think his body will continue to develop, but probably more in the lines of a Channing Frye. JaJuan's problem isn't that he's afraid to go bang inside; it's that he's not going to be able to do it at the pro level with regularity because of his stature.
He's had plenty of games where he hasn't been afraid to mix it up with a more physical big man. He's just more talented than they are most of the time. He will actually post up to turn and place a hook. He'll get a guy on his hip and use his quickness to slip off for an easy dunk. He's got an OK turnaround with a guy on him. He's long-armed and has a pretty high release point, so he'll be able to get some shots off.
His real asset off a bench will be an ability to alter shots from the weakside. There should be a role for him in the NBA for that. He'll have to develop to become more than a shot-blocking jump-shooter, though. But guys that size with those skills are rare, and he should get drafted in the back third of the first round.
Oh, and he rebounds like Jermaine O'Neal. All arms and hops.
And since someone brought him up, E'Twaun Moore should be able to make a roster as a second-rounder. One of the more frustrating players to watch as a Purdue alum, because he's either on or straight broke. The difference this year is that he's defending at a much higher level.
E'Twaun's biggest issue is that he's at best an average ball-handler. He won't be able to CDR his way through NBA defenses like he does the college ranks. But he's got a knack for getting his shot off in the lane and can hit an open jumper. Smart player who's becoming more and more willing to dive for a ball. He's Lance Stephenson with some polish and a brain.
Worst thing about E'Twaun: He's a terrible clutch-time free-throw shooter.