Just mainly reiterating what Roy already nailed down, but consistency is definitely the C's biggest weakness, and I think it's mainly from the lack of history/playing time with each other. The more games we play, obviously, the better it gets, but we're still a ways from being consistent, at least on a consistent basis, that is. (Sorry) This team is so up-and-down, (or hot-and-cold), and can be playing huge on both ends for ten minutes, then have these complete lapses and totally let down ... it's extremely frustrating to watch.
The next biggest weakness I see is shot selection, or I should say play selection, to be more accurate. This team seems to have a propensity to be a bit lazy, and not continue to attack the basket, even when it's proving to be successful, (which is most of the time). Everybody loves to hit the "Big Shot" and is plagued by the temptation to do so, but we fall back on it far too often, and choose to take a three, even when there's a clear lane to the basket ... it's truly baffling at times.
Following on the list for me would be foul-shooting. This drives me completely nuts, and there's really no good excuse for it. Free-throws are Basketball 101, and for the best team in the NBA to miss so many foul-shots, especially in the playoffs, when every point is crucial, is just absurd to me. Honestly, if I were Doc, I'd spend way more time and energy getting these guys to tighten up from the stripe. Maybe he does already, but it sure as heck isn't enough. These guys should be a bit embarassed about it, and take it upon themselves to do much better. They could have had a much easier time this season, if they'd just hit more of their free-throws.
Fourth in line, I feel, is foul trouble. There have been many games this year, including the post-season, where two or three of the starters, (usually KG, Perk, or Rondo), have been sitting on the bench with two fouls by the middle of the first period. This is really unacceptable, and while the poor officiating has had much to do with it, we should be taking that into consideration, and minimizing the damage as much as possible. There have been so many games where we came out and got off to a great start, then KG and Perk got two quick fouls, and the wind was totally gone from our sails. It kills the momentum, and changes the face of the game.
Keeping it to five, I'd have to say lastly also, that it's the coaching. I think the Detroit series was a step in the right decision ... Doc made some great adjustments and called some very smart offensive plays, especially halftime changes. But unfortunately, there were still times when it was painfully obvious that the wrong players were on the floor, and the match-ups weren't right. Eddie House especially, should be the first one off the bench for Rage, not Sam, and Leon should be used far more than he has been. I also think that the "two-big" or "three-big" line-up should be used more for the subs, especially with LA, and that Sam should be used more as a two-guard, and play with Rondo, not in place of him.