Defensive efficiency.

Matt Yglesias:

Before the season started, I thought the weakness of the Boston Celtics was going to be defense. Ray Allen seemed like a clear liability, Paul Pierce is unimpressive, and look beyond the "big three" I didn't see much of anything to help out on this end. Kevin Garnett, obviously, is brilliant defensively, but it's a team game. Boy oh boy was I wrong about that. Boston's defense is not only the best in the league, they're by far the best in the league, a fact that may be somewhat obscured by the fact that they play a middling pace whereas San Antonio and Detroit go at a crawl.

The chart plots points allowed per hundred possessions relative to the league median. The very worst teams in the league -- New York and Minnesota -- both allow 5.7 more points per hundred than the median. Boston, by contrast, allows 5.5 fewer points per hundred than does the next best team. In short, not only is Boston the best, but the gap between the best and the second best is enormous. They're leaving everyone else dead in the water.