The NFL is boring. One of the arguments used to be that players did not move around, and you knew who "your guys" were, unlike college that turned over every four years. Staubach always QB'ed the Cowboys, Bradshaw the Steelers, in fact, most of the rosters in the 70's and 80's were pretty stable, trades and player movement were somewhat rare, that all changed with free agency and each team has a core of 10-15 players, the rest are on a permanent merry go round.
The second killer is parity. If every team can beat the other on a given Sunday, there is no longer any such thing as an upset. The homogeneity of the game now makes every team and every game look about the same. Dull and duller, I cannot bear to watch it anymore, especially when the action stops every 2.5 minutes for another round of commercials. I haven't watched a full NFL game live in probably a decade.
I won't even go into the stuff I have seen behind the scenes that turned me off...