Welcome to CB!
Boy - I wish Bradley became a Russell Westbrook type player this offseason, but it will never happen for a few fundamental reasons. The first being the player's mentality/mindset. Westbrook came into the pros with the reputation of being a scoring guard while Bradley's rep was defense. It can be difficult to change your mentality on the court. We have seen this with Jeff Green. After going to Georgetown (pass heavy, team oriented offense) and deferring to Westbrook, Durant in OKC and Pierce, Garnett, Allen, Rondo in Boston - it has taken him a considerable amount of time to develop the instinct to be a primary option (to be honest, I still don't know if he has).
The second thing holding back could be fixed with practice, but it is critical - ball handling. Bradley isn't a strong enough ball handler to beat defenders of the dribble and get to the basket.
Lastly - as others said - Westbrook is among those from an elite, hyper athletic gene pool that others could only hope to be from. Bradley is quick, but he is no Westbrook.
To be an NBA player you must do one thing exceptionally well. The great players can do multiple things exceptionally. Bradley has made his niche as a defensive stopper, but he still has room to grow. I'd like to see him come back this season with an improved handle on the ball, improved jump shot, and improved court awareness. Hopefully working with Stevens and Rondo will help his awareness on the offensive end.