From a football standpoint, the Big 12 has been vastly superior to the ACC over the past decade. You could even argue that the Big East has played better football than the ACC since BC, Miami, and VT left. The Sagarin ratings of the conference support that view. If Florida St and Miami had been close to their teams of the 1990s, the ACC would have been in a better position. VT, which is the perceived conference football power, has had one top 5 finish and that was is 1999 when they were in the Big East with Michael Vick.
Here are the top ACC teams in the final AP Poll:
2011: VT - 21
2010: VT - 16
2009: VT - 10
2008: VT - 15
2007: VT - 9
2006: Wake Forest - 18
2005: VT - 7
2004: VT - 10
2003: Florida St. - 11
2002: NC St. - 12
Clearly, the ACC missed the boat when the Big 12 was rocky and should have pushed to add Texas, Texas Tech, Oklahoma, and Oklahoma St. to be the first conference to go to 16.
Outside of poor football performance since the first expansion, the ACC had three major problems:
1) They locked into a LT contract with ESPN right before more media companies became interested in buying rights. (Believe it or not, the BE had an advantage with their contract coming up for bid, but they failed to capitalize on it, at least so it seems.) ACC's contract is worse than Big 1G, Pac 12, SEC, and Big 12. That's a problem for FSU and Clemson who compare themselves to their main rivals, Florida and South Carolina.
2) Too many private schools. The ACC has by far the most private schools of any football conference, 5. In a world of conference networks, this is a liability as it is more difficult to get markets to play top dollar for a conference network for small private schools. ND and possibly Syracuse would be an exception, but there is no way the Boston market is paying top dollar for an ACC conference network.
3) Too many North Carolina schools. Having 4 schools from North Carolina is not good for media contracts. How does Wake Forest add any value to the media contract?