The one thing holding Brown back is his (lack of) basketball IQ. He is an elite athlete, but he doesn't possess natural basketball instincts.
I think we may agree, but I don't think I'd call it basketball IQ. I think he's an extremely intelligent player. I also think that any success he has on the court is the direct result of his work ethic.
On offense, I don't think he quite has the "feel" of scoring in a reliable way within the rhythm of the offense. He scores--in bunches sometimes. But it seems like his points don't come as easily. I also think that, while he is a good passer, his feel for passing with CBS system has not been great.
On defense, I think he's really smart, but I think he takes plays off. He has the potential to be an all-NBA defender, but he doesn't always seem locked-in. You see the potential when he goes against tough matchups or the best players in the NBA. I just wish I could see it more.
With his work ethic and the new opportunities within the Celtic offense, I would guess he'll take a step forward. He might have a slow start to the season and be a bit inefficient and turnover-prone (ala Tatum last year), but I think by the end of the season he will be improving.