The Pats have been good for a decade but this season, they've shown major weakness. Like giving up tons of passing yards to everybody. The 3 times they've won the SB, their defense was one of their strengths. The other times their defense has sucked, they've gotten bounced.
The last few weeks, they've been down ~20 points because the offense seems to start slow with Brady's shoulder injury. Sure, they've come back against these weaker teams but against a solid playoff teams, like the Steelers, coming back 20 points is not easy.
Everybody wants to harp on the D for yards allowed. Yes, in that stat they are terrible. Where they aren't too bad is in points allowed. They are are right there with the Packers and Saints. Also, they are #2 in interceptions, and ranked high in overall turnovers as well.
They certainly have defensive issues, but if the offense is playing well, they will do well. The other high scoring teams in the playoffs have not-so-great defenses as well, and the good defensive teams have pretty weak offense.
I think it will be a rough road for them, but they are certainly a contender.
edit - and yeah, that's the one thing I see being a problem. If they come out slow to a good team like Pittsburgh, it's gonna be a lot tougher to come back. The Steelers are pretty banged up going into the playoffs though, and barely got past the Browns.