I think what did him in the last couple years was that he completely went away with what won so many games.
He kept putting an old, injured McNair out there with average receivers and tried to show everyone how he was an offensive genius. Boller is not good, but he is way better than McNair at this time.
And he refused to use both strategy as well as a $20-million stud RB several times and it cost them a lot of wins.
I realize the D was banged up right from the start this year, but overall I think Billick's ego did him in and he needed to go. The offense is now going in the right direction with Troy Smith and that D has a couple/few good years left. They need a good WR to complement Smith.