Well first you need the All-22 film, which until very recently wasn't available to the public, even paying subscribers. It now is (though think you need to pay a fee to access it), otherwise you only see what the TV camera sees and often can't see how Safeties and CBs like up, etc...
From there you just need the manpower to analyze every play of every team in every game.
There are a bunch of groups out there doing this - Aaron Schatz's Football Outsiders is one of them.
http://www.footballoutsiders.com/