This defense is better than last year's. By a wide margin.
My favorite way of looking is per possession, of course.
Points Per Possession [Defense]:
2011 - 1.9 (21st)
2012 - 1.7 (12th)
Yards Per Possession [Defense]:
2011 - 37.5 (32nd)
2012 - 32 (21st)
Turnovers Per Possession [Defense]:
2011 - .177 (3rd)
2012 - .215 (1st)
Drive Success Rate [Defense]:
2011 - .731 (30th)
2012 - .690 (15th)
And the Patriots offense had more cracks at it this year, too:
Offensive Drives:
2011 - 173
2012 - 180
In fact, if you look at the offensive efficiency stats, the 2012 offense is damn-near identical to the 2011 one.
Offensive Points Per Possession:
2011 - 2.79 (3rd)
2012 - 2.82 (1st)
Offensive Yards Per Possession:
2011 - 39.5 (2nd)
2012 - 39.2 (1st)
Offensive Drive Success Rate:
2011 - .775 (2nd)
2012 - .787 (1st)
The conclusion I would make is that while the offense improved nominally, continuing it's elite efficiency at an extremely high level, the defense improved noticeably. As a result, the offense saw the ball a little more, and scored more in total.
Before this season, the defense had been a detriment to the team's success for the last several seasons. It's metrics were god-awful, and they were only slightly redeemed by causing turnovers at a high rate.
Now, between the improvement in its efficiency and the even further improvement of turning the ball over, the defense - for the first time in a long time - is actually helping this team win.