I again go back to this point: JaJuan Johnson may never have what it takes to be a star, but does he have what it takes to be a longer Brandon Bass? I think it's quite possible.
He hits open jumpers, he hustles, he finishes around the rim, and he at least tries on the boards and on defense. These are are attributes of a strong role player, which players like Kurt Thomas and Brandon Bass have made careers out of.
On the other hand, people are seeing a guy like Dionte Christmas take over some summer games, but to me, that's potentially less useful than the attributes above. Only stars are allowed to take over games and jack up crazy shots. The fact that a guy who (at best) will be the 15th player on this team can do it doesn't necessarily matter. Doc will never give him those looks in the game. What does matter is that someone like him can stand on the weakside for 20 seconds of the shot clock and then bury the corner 3 as the clock expires (see Eddie House, James Posey, and Mikael Pietrus).
So again, I wouldn't get overly down on Johnson yet. I don't think he'll ever be a star. But given that he is a big man with an actual shot, has the athleticism to finish around the basket, and has decent length, I think he can succeed on this team. He already had a couple double digit scoring games last year and many, many players have proven over the past 5 years that if you can hit the open shots given to you by playing with the many stars on this team, you can be successful.