Green Bay's defense gives up ridiculous amounts of yards. They can't stop anyone unless the get an interception (which they do often). They also have the worst prevent defense in history, and they play it a lot.
I am a life long Green Bay fan, but I don't have a good feeling about them repeating. Offensively they are a juggernaut. Defensively they suck. Sure the NFL has turned into a pussy league designed to allow passing, but defense still counts for something.