A lack of strength. Mental problems. The skill is there but he just doesn't play with enough fire or aggression.
The mental thing has to be there to begin with, IMO. With foreign players, I think a lot of GM's just assume that the national team commitments of international prospects is enough for their development. But a lot of these players need a solid season of individual training to truly adjust to the NBA. Bargnani added 12 pounds of muscle this offseason working out with Jermaine O'Neal and a bunch of personal trainers. I'd like to see the Nets devote the same sort of attention to Yi.