Outside of injuries, the big question is still how they come together defensively. LeBron's a very good defender, but we'll have to see about the rest of the squad.
Achille's heal may be point guard penetration, and a lack of a a real enforcer/shot-blocker in the middle. If Joel Anthony and Big Z are holding down the fort, that's not exactly like Perk/KG or like Dwight Howard.
Not sure how they're going to stop Howard, anyway. Joel Anthony going to get the job done? I like Z, but he's not playing more than 20MPG and he's slowed WAY down.
If they bring back Arroyo or go with Chamlers as the primary point guard, Rondo could have a field day. Can't stop him from penetrating, and don't have anyone to really back them up inside. If Wade's playing the point, I'm not sure he's got quite the quickness to deal with Rondo's first step either.
Be interesting to see if Bosh is willing to use his athleticism more on the defensive end, to use his quickness and leaping ability to be more of a shotblocker in the middle. Whether he's willing to change his game, focus more on that end of the floor and let Wade/LeBron do most of the scoring, is another potential friction point.
Two of the teams they have to go through; how are they stopping Dwight Howard, and how are they stopping Rajon Rondo? Miami Heat are still the team to beat, but their potential weaknesses are against those two players.
EDIT: I keep forgetting about Haslem. LeBron ought to be paying him under the table, he's giving up a crapton of money to go back.