Here's my list of reasons we lost, in order:
1) Shaq (Center position) Jermaine played great, but too few minutes for a starter and there was no backup. Davis played 58% of all available minutes to him during the regular season, and too many in the playoffs. The statistical downgrade from Shaq to Davis is nothing short of enormous. (Game 3 is a good example of what we saw in the regular season too).
2) Rondo's injury. Because of the lack of depth at PG he still had to play and wasn't nearly as effective.
3) Miami Defense / Celtic Coaching. Miami played great help defense, and Doc did nothing to mitigate this. In addition, Doc let Miami's "small lineup" dominate w/o trying to exploit it.
4) Ray Allen / Lack of Screens. The screen setting was terrible for Ray, which made it much easier for Wade to stick on him. Offensively Ray shot well.. when he shot (he didn't shoot enough). Defensively Ray was the primary reason Wade (as usual) dominated the Celtics.
5) Wade. He played great defense (for once) and great offense. Unfortunately in a game that had physical play go w/o a whistle, Wade also got to the free throw line if someone breathed on him. Double standard.
6) Pierce shrinking from the moment. He lost his composure in Game 1, had costly turnovers in Game 5 towards the end, missed a game winner in Game 4, and routinely disappeared at times. He didn't look like himself.
7) Lebron fully accepting the facilitor role and stepping up in games 4 and 5.
8 ) Bench play. This was supposed to be an advantage for the Celtics, but it turned out to be a detriment.
9) KG's sporadic play. Some games he took it to the hoop with great success, and others he settled for jumpers (and made only 1). His effort was 100% there, but his shot and shot choice wasn't.
10) Jeff Green. When he attacked the hoop he looked great. Unfortunately that wasn't often. His defense on Lebron wasn't bad, but he had a propensity to foul. And his turnover(s) in Game 5 proved costly. Green was supposed to be the wild card advantage in this series, but he wasn't an advantage at all.