In many ways, you guys are all hitting on aspects of the truth. There are two extremes here:
Extreme #1: Losing the Super Bowl is the defense's fault.
Those who argue this point to the fact that the Giants, on their last possession, just carved up the Patriots' D, when the Pats had a chance to hold them for the win. Whatever else happened during the game, the D had a lead to hold and could not get the job done. Eerily similar to SB 42 and the Giants/Pats regular season game this year.
The problem with this view isn't that it's not correct, because it is true that the Pats' D has been notoriously bad at making game-ending stops. But the reality is that the D isn't very good to begin with. It's better than it was early in the year as I've documented, but it still isn't very good. And the fact is that they held the Giants' offense - which was coming into the game averaging 28.2 points a game over their previous 5 contests - to a mere 19 points. In other words, the beleaguered Pats' D held a high-scoring offense to 9 points under their average (I'm speaking of the G-men's last 5 games, when they got hot...that's a relevant stretch of games). There aren't too many people who would have expected NE's defense to hold up that well. And they did so without a lot of help from the offense - which had numerous very short possessions, forcing the defense to be on the field much too quickly. Normally, if the D is going to give up just 19 points, that's a recipe for a big Patriots' win. But not when the Pats' offense only scores 17 points.
Extreme #2: Losing the Super Bowl is the offense's fault.
This view seeks to absolve the defense of blame based on what I just said at the tail end of the previous paragraph. Hey, they did a really good job overall containing a pretty explosive offense, and gave the offense more than enough chances to win. They did their job and this loss is on the offense.
The problem with this view is that the Pats now have a pretty serious history of giving up late scores. The fact is, despite how they played for most of the game, they did have a chance to stop the Giants at the end and they simply didn't get it done.
So both perspectives are right to a degree and both extremes are to be avoided. Both units had chances to make plays to help the Pats win the game, and while both did some good things, neither unit made enough plays and the Pats did not win.