The Spurs really are the only team that can beat the Heat due to their roster flexibility in playing big or matching Miami's small ball. They also have too many damn 3 point shooters.
Miami also plays super aggressive pick and roll defense and we've seen teams get loads of 3 pointers. Spoelstra is slow to adjust in a series and he is very stubborn. Heat fans will co-sign me on this.
SA is also a good defensive team in their own right with post presence and the young guys on the perimeter playing D. Ginobili has had a resurgence too. Pop won't start Splitter but he will have either Duncan or Splitter on the floor all game to protect the rim. Diaw is an X-Factor because he can shoot, create off the dribble, defend and he can also post up the smaller 4s of MIA.
LeBron and D-Wade cannot be stopped. But Leonard and Green play good enough to make them work for their shots as we saw in the NBA Finals last year.
The Thunder are also a good defensive team but Scott Brooks doesn't put his players in position to succeed especially vs. Miami because he refuses to match the small ball lineups and their defense suffers. OKC has no problems scoring as we saw in 2012 but their problem is defense and their coach not adjusting correctly. If he would just let Perry Jones and Jeremy Lamb play with Adams and/or Ibaka, it would be interesting but he didn't let some of these young guys play all season

It's such a damn shame.
Also, Russ and KD throwing up horrible shots are just like turnovers and that's lethal with MIA's fastbreak. If they get into ISO ball which they've done all year and postseason, the Heat will salivate at that opportunity because they know it means easy baskets the other way with those long rebounds.