There seem to be quite a few people who have very high expectations for Pietrus here (not specifically this thread, just a general observation, there seems to be a lot of highly optimistic Pietrus posts on Celticsblog).
I think those expectations are too high and that they should lower them some.
Pietrus is a fairly unintelligent offensive player who is known as a detriment to ball-movement (ball stopper). His scoring efficiency is routinely mediocre due to his suspect shot-selection. And he is a poor passer.
Offensively, Pietrus is a very streaky player with poor decision-making skills and it's those poor decision making skills that have stopped him from fulfilling his potential (as an elite role player) everywhere he has been.