Good:

He's got excellent size and athleticism for both forward spots. He's a ridiculous 257lbs, but he's really athletic, posting good times in the sprint and agility drills, bench pressing a ton, and jumping out of the gym.

In short, he compares very well physically to plenty of successful players. The closest contemporary comparison might be Ryan Gomes, but with way more athleticism.  Perhaps Lebron James if Johnson loses 10-15 lbs. Yes, really.

He's a trained martial arts expert, which suggests to me excellent body control and the potential for good discipline.

Productively, he's demonstrated good but not great returns to his unworldly physical abilities. First, Wake Forest played at an NBA Pace. That's something that's not generally noted, but it's pretty important when considering how guys adjust.  For example, Blair and Pitt played a game that had something like 80 possessions per 48 minutes. The slowest paced NBA team, the Blazers, played 86.6. The Bulls played 93 I think. Wake Forest played about 90 possessions per 48 minutes. So he might be a bit more used to that sort of play from the get-go.

Within this background, he produced at good but not great levels. He's got range out to the 3 point line, he can rebound, and he finished pretty well. He appears to be able to pass the ball.

Bad:
The only physical negative I see is that weight. He's got 12% body fat in a world where the average successful NBA SF has 6-7% and the average PF has 8-9%.  With proper conditioning, I'd guess his proper playing weight would be about 245lbs.

Statistically, he was right on the margin of "troubling" in two areas. First, he fouled quite a bit, though he's not a foul machine. Second, he was just under 70% as a free throw shooter, which isn't all that great, but marginally acceptable. Third, for a guy with his physical tools, he really didn't get to the line a lot compared to what really top notch guys do.

Other:
* He apparently annoyed Chris Paul so badly that Paul told the Hornets not to draft him if available. Usually good players want their college teammates, especially if they think they can be good pros. That's sort of annoying.
* He's talked about an MMA career if basketball doesn't work out. The upside of that is he's a bad ass. The downside is I don't want him thinking aobut what happens if basketball doesn't work out.
* He came to basketball a bit late. I think I read he only started playing a couple years into high school.

Conclusions:
James Johnson has the tools to be an absolutely spectacular player. His physical gifts are second to none, even for the NBA. And his skills are impressive. He's demonstrated every tool you'd want from a basketball player on the court. Except... an overwhelming commitment and drive to succeed.

That's the common theme.

Physically he's gifted, but he's not in the best shape he could possibly be in. Productively, he's demonstrated superb skills, but hasn't produced to the level one could imagine a player with his skills producing at.  In short, it's all there, and even if he half-asses it, he can still be useful. But if he really commits and works his ass off, he could be amazing.