The biggest reason why the team fell out of the playoffs was Mike Tannenbaum.
In a very short offseason, Tannenbaum signed Antonio Cromartie and Santonio Holmes to multi-year deals, where only Cromartie prove himself under his new deal.
He also signed a fresh-out-of-prison wide receiver named Plaxico Burress and ageless wonder Derrick Mason, both of whom clearly prove that they were over the hill, instead of keeping 2010's top wide receiver, Braylon Edwards, leaving Mark Sanchez to try and connect with a wide receiver who clearly didn't complement him well.
What he didn't do was be more aggressive during the offseason, losing out on weapons like Darren Sproles and Nnamdi Asomugha. Had he signed players like them, the Jets might have had a better year.
Tannenbaum's tinkering and tampering with the team cost them quite dearly, and while it doesn't look like Woody Johnson will fire him, he certainly deserves plenty of the blame associated with the failure of the 2011 season.