Cleveland and Orlando are front running teams. When the going gets tough, both of them fold and this is directly linked to the attitudes of LeBron and Howard. Neither hold their teammates accountable and neither has displayed the ability to inspire them, either through words or through play.
Boston is very different. Allen, Garnett, and Pierce have all taken turns being the big time clutch guys, and Rondo has trumped all three of them. To me, Cleveland and Orlando have similar talent levels/athleticism to the C's, but neither of them have the mental fortitude to compete with Boston.