There are two answers to this very simple question:
One answer is: "Nothing, Idiot."
If the NBA did a survey of each team and asked them which team they'd least like to face in the play-offs -- I'm pretty sure The Miami Heat would be the overwhelming leader on that list. The Heat are the Champs and any second-guessing from the media talking heads is only for the purposes of having something to talk about on TV. The Heat have what it takes to win it all. Championships are won in June, not in January and February.
The Second Answer is: "They are tired and lack regular season motivation."
I've been saying for 3 years that the regular season doesn't really matter for the Heat. For 3 years+, this team has been playing in sold-out arenas and have a huge target on their backs every night. They've proven over the last 3 years that they aren't really to be questioned. (Won 11 out of 12 play-off series in the last 3 years). They're veterans and they really felt the grind of the 27 game win streak last year. What they're doing could possibly be viewed as "over-correcting" at years end ... but again ... from where I stand they've earned the right to be trusted.
This said - I do think it''ll be really tough to get through both the Pacers and Thunder (my prediction for West champ) come May/June. Those teams are STARVING (they were hungry the last 2 seasons) and I'm not sure the Heat are gonna be able to find the extra gear when the time comes. Nothing about the Heat's regular season performance is the reason why I think that, though. It's more what I'm seeing from the other teams. They are deeper and more experienced than the past and they both have players who can shut-down LBJ when he turns into a one-man show.
I still believe the Heat can 3 peat. That would be the obvious best outcome - especially considering the contract situations of the big 3. No way any of them walk away from going for a 4th in a row....
For now, "nothing is wrong with the Heat." They're doing what they need to do and I'd prefer not to talk about it until the play-offs begin...