Don't get me wrong - the Jets were the best defense in the NFL last year and a very good defense, but they were not a game-changing DOMINATING defense. In fact, in a lot of games, they were unable to get a key late stop to win the game - they gave up late scores to lose close games against Miami, against Jacksonville, against Atlanta, I think even against Buffalo. The only reason they were able to get past San Diego in the playoffs is because Nate Kaeding self-destructed. Dominating defenses make those stops - dominating defenses win those games, and the Jets defense was not a dominating defense.
They've added more big name players in the offseason - but Cromartie's reputation was built in his rookie year and he hasn't been as good since - Jason Taylor is on his last legs, and Kris Jenkins hasn't shown he can stay healthy. Plus, it remains to be seen if Revis can play at the unconsciously good level he was at last year - if he reverts to the mean even a little bit, it will have a big impact.
Again, I'm not saying that the Jets defense isn't good - but it's not the special defense that can carry a team to a championship without getting help from the offense. Mark Sanchez is going to have to improve by leaps and bounds this year for the Jets to live up to their hype.