Here's what it is. The Pats defense is obviously good enough to get them to the AFC Championship game...this team goes every damn year. And they've got enough solid personnel on defense where the numbers are going to be pretty good.
Comes down to one thing. Can they make a stop when they have to. Can somebody make a play. That is the question, and that has been the concern to an extent even this year, in games that were pretty comfortably in hand because of the offense (Pittsburgh, Buffalo), can somebody make a play on 3rd down? That's it. The team has parted ways with three guys who, to varying levels, were able to do that, from every now and then to sometimes: Easley, Jones, and Collins.
If the Pats fall short of the Super Bowl (assuming decent health) there will be a whole bunch of finger pointing, but 99% chance it will come down to the defense not making a play when it has to.
Note that I think the Pats are winning it, in part because I simply can not envision what other team would.
edit: oh, and look forward to watching Lewis play again.