WHAT KEPT THE PATS FROM BEING UNDEFEATED?

The Redskins and this man.

Just note that the Pats crushed conventional wisdom (and sophisticated wisdom):
1) They didn't have a dominant running game - 100.4 yards per game was 2nd worst in the AFC.
2) Their results were consistently better than their stats would lead you to expect, as calculated by the folks at Football Outsiders.
3) Conventional wisdom among statisticians is that when you see a team do 2), their W/L record will regress to the mean. It didn't.