Some folks (myself included) will point out that the defense wasn't that great in the 1st two thirds of the season.
The bigger problem, though, was that Brian Kelly insisted on using all of that offensive talent to run his dink 'n dunk passing offense, along with making Dayne Crist run too many designated QB runs. Had he simply let the team run standard dropback passing plays and throw the ball downfield, that offense would have been far superior.
Crist did what he could in that non-optimal offense, and actually came away with decent numbers until he got his knee blown out in the Tulsa game. 59% completion, 15 TD, 7 INT.