You see the impact of a quarterback time and time again in the NFL. Obviously we know what happened in Tampa and New England last year, but they are far from the only example.
Look at the Colts. Years of winning 10+ games, Manning gets hurt they go 2-14, they draft Andrew Luck and then immediately go back to winning 10+ games.
Broncos were 8-8 with Tebow, sign a much older coming off an injury Manning and go 13-3. Same coach, basically the same core, only major difference getting Manning.
Rams go from crappy, sign Warner, go 13-3 and win the Superbowl.
I mean heck just look at the Niners record when Jimmy G plays vs. when he is out since he joined them. Astronomical difference.
I agree with the idea that in general an individual football player doesn't make that much difference, EXCEPT the QB. The QB is the most important position in all of sports. It is almost impossible to win at a high level with even a mediocre QB, let alone a bad one, and that just isn't true of any other position or frankly coach. A coach isn't winning without a top tier QB, except in uber-rare circumstances (there are obvious exceptions).