I think the best way to describe the Heat defense is that they have multiple, extra long Rondos. They are nowhere close to the most disciplined defensive team. They make a ton of mistakes, overplay everything...but they are insanely aggressive, and have the athleticism to recover. And then, most importantly, they have the talent to convert on just about every mistake they force.
I think this defense can certainly be beaten however, just like Rondo can be beaten on defense. But it is not easy, and you need to be very crisp, which this team just hasn't been so far.