Is Johnson ever going to be a star? Perhaps not. But I would caution two things.
First, more than half this board deemed Avery Bradley a complete failure last year, with many claiming E'Twaun Moore to be better than him. Given that Johnson has yet to even have a full training camp with the C's, I would take a wait and see stance on him rather than leap to conclusions about him.
Second, part of the complaint about him is that he isn't dominating summer league games. To that, I would say that isn't necessarily a bad thing. I really question this notion that summer league stars translate to the NBA game. First, we've seen many guys dominate summer league games and be busts in the NBA. I am looking at you Joe Forte, Kedrick Brown, and Jerome Moiso. Second, most of thr guys who take over summer league games won't be allowed to do so in the NBA. Instead, they'll be asked to hit open shots, make passes, and play defense.
Now, I am not claiming that JJJ is setting on the world on fire in these areas, but he has hit some open jumpers and has played some decent help defense. If he can do that in the NBA game and pick up his rebounding, he can be an effective player.
That said, if Boston trades him, I won't lose sleep. I don't think he will ever be a star. But I do think still has the upside to be a longer version of Brandon Bass. And that's not such a bad thing.