Interesting compilation of numbers here:
http://thecomeback.com/nba/boston-celtics-elite-defense-discipline-despite-roster-turnover.html. In terms of team performance, the summary is that the Cs are winning by playing elite defense, by not turning the ball over, by rebounding and getting to the line at a good clip.
A couple of the very interesting stats in the article. The Cs are 26th in pace this year (down from 13th). Most of the Cs guards have been really good at not turning the ball over, particularly Kyrie, and the Cs have only 14.5% of possessions end in a turnover, which is 4th in the league (note that Al, Tatum, and Smart have been in the lower third of their position in turnover %).
Pretty amazing. Rest of the article is worth a few minutes.