I'm going to really have to go into the lab and break some things down before I could make a realistic prediction for this series. I will say that, as others have alluded to, the Heat are the team that scares me the most in the East and are right there with the Lakers as far as teams that worry me. But, a few of my initial thoughts:
1) The Strengths/weaknesses match-up between the Heat and Celtics seem to favor the Cs. Said another way, I think the Celtics are more built to blunt the Heat's strengths than vice-versa and that the Celtics are more built to take advantage of the Heat's weaknesses than vice versa.
For example, the Heat (obviously) have huge talent in the iso and are outstanding when they can get out and run. The Celtics' defense, on the other hand, is built to prevent iso scoring and to slow down fast breaks.
Then, the Heat's defensive strength is in their athletic 2 - 4 positions that hide their point guard and allow their centers to not have to do so much to anchor the defense. The Celtics, on the other hand, have a point guard that gets to the rim and bigs that can take advantage of donuts (think what Rondo and KG did to the Cavs last year, which is compounded even further if Shaq can come back and contribute a few minutes).
Those are just examples, but I think in general the Celtics are more built to defeat the Heat than vice versa. Of course, none of that means for sure that the Celtics actually DO beat the Heat...but I think they should be favored. Right now "Celtics in 6" seems like a reasonable default prediction.