Davie and Willingham had pretty good rosters, but were so limited in their coaching methods that the offenses never got any traction under them.
Davie was bad, but at least he got us to a Fiesta Bowl. He put too much trust in Kevin Rogers, though, and could have had so much more had he put Urban Meyer in as the OC.
Willingham was especially bad, trying to make a power running team run that stripped down, super-predictable version of the West Coast Offense, combined with a quarterback who was only a pure runner.
Charlie Weis had two classes of reasonable talent (2002 and 2003), along with a smattering of the 5th years from 2001. There was very little talent in the 2004 and 2005 classes, which resulted in that awful 2007 season. While there was enough upper classmate talent for the 2005 and 2006 seasons, that was all that was available. Had he simply kept anyone else in the defensive coordinator's position other than Jon Tenuta, that would have been a BCS bowl season.
Even Nick Saban or Urban Meyer wouldn't have been able to get to .500 with that 2007 roster, and that 2008 roster was still lacking in terms of developed junior and senior talent.