You always hear the phrase "star driven league" tossed around when generalizing about how to win in the NBA. Well, its true. The two best players in the league are still competing in the playoffs because they are elite.
But, perhaps its less the "star" and more the "elite." Outside of the Heat and the Thunder, you have two teams that also have elite qualities. The Indiana Pacers display the league's most elite defense. The Spurs host the league's most elite system paired with the most elite coach.
Therefore, you could argue that being elite at something is your ticket to contention. No average teams have made it. No third tier teams have made it. Only Elite.