According to David Glenn it is the attrition of personnel for various reasons, some of them not so good.
Bunting basically has a hole in the middle of his program. He has lost so many players via attrition, for so many different reasons, that he finds himself playing catch-up in terms of personnel on an annual basis. There are basically two ways to build a winning program in the new ACC: (1) consistently recruit significantly better talent than everyone else, a la Florida State or Miami, and/or (2) consistently recruit "good enough" talent, then retain it, develop it and coach it up on game day, a la Boston College and Georgia Tech. (Virginia Tech and Clemson are combining #1 and #2 pretty well right now.) Under Bunting, the Tar Heels have accomplished neither goal.
Glenn goes on to point out that UNC has lost an astonishing 35 players from 2003-2006 to everything from early graduation to felony arrest. Undoubtedly some of these players would be experienced starters in a team that is lacking experience in key areas.
First of all, David Glenn is a freak for his ability to come up with this kind of data and analyze it so perfectly. Secondly, as much as I focused on the results of Bunting's tenure Glenn has provided some of the why and that is mishandling personnel both in terms of losing them and also by failing to identify players who would be prone to attriting earlier for whatever reasons.
At any rate that still falls on the Bunting to fix and I am not sure that he can fix it because it seems like he will be forever trying to play catch-up. Also, how long will it take Bunting to address these personnel issues? Probably longer than Tar Heel Nation has patience for at this point.