Allow me to head-off any commenters with my first few sentences.
Yes, the New York Giants won the Super Bowl, but that was last year, and a lot has changed. Yes, the New York Giants have an amazing pass rush, but this is a ranking of the entire defense. Yes, Jason Pierre-Paul is really, really good.
There, now let's get down to the actual football talk.
The Giants defense lives and dies on the pass rush, and so far, that's working. What should be concerning is what happens when teams scheme around the pass rush, because it's going to happen. As talented as JPP and Co. are, someone will scheme against them, and they'll have to rely on the other seven defenders to win ball games.
The linebacking corps wasn't good in 2011, but they were covered up by a pretty solid secondary and that great defensive line. In 2012, much of the same. Mathias Kiwanuka is a beast to handle, but he isn't a help in coverage. Same with Michael Boley, who is a good run defender but bad on third down. The inside is solid enough with Chase Blackburn, but he's not someone you work your game plan around.
The secondary is going to be a question with Terrell Thomas out and Aaron Ross now in Jacksonville. Sure, Prince Amukamara should take a step forward now that he's healthy, and Corey Webster is a good option, but neither player can shut down Dez Bryant or DeSean Jackson.
As for the safeties, well, they're overrated. Or at least Antrel Rolle is. And badly. Rolle is a "name player," but he makes no plays on a consistent basis. Watch Rolle against the run or in coverage last year, and you'll see a player routinely out of position and often playing from behind. Kenny Phillips can be good, but he's too often caught trying to make far-reaching plays to cover for Rolle.
The Giants defense will swing from dominant to average, depending on the offensive line they face and the ability of the play-caller on the other sideline to use their aggressive defensive line against them. That adds up to a great defensive line, but an average overall defense.