I think that a big part of the secondary's failure earlier on was poor play by the safeties as a whole. As a matter of fact I'd go as far to say that poor play from the safeties likely plagued us for awhile, and may even have contributed to McCourty's falloff in the 2011 season too, but I have no hard proof of that.
Wilson just wasn't getting the job done and is too green at the moment (proven by spending the rest of the season in BB's doghouse after making the exact same mistake twice in 2 games that led to long bomb TD's), Gregory was just starting to get used to the assignments and responsibilities here in the beginning, and Chung just isn't too good in pass coverage/carrying out his assignments and reads/maybe even all of the above.
The improvement of the safety play is the biggest turnaround for this pass defense, and that isn't knocking McCourty at all because I think he has played great back there--tremendous actually, it's just stating that I think that safety play in and of itself was an enormous factor on this team as a whole, and has trickled down throughout the CB's too for obvious reasons.
I would still love to know if the reports from Bedard and SMY earlier in the year are true or not, as to whether the team wanted McCourty back there all along at FS? The claim is that Belichick just wasn't happy enough with the depth at CB during training camp, and that's why they couldn't start the season with him playing FS.
If that is true I wonder if the Dowling situation of the poor play and injury combo didn't once again catch them off guard a bit, as they likely had a bigger role in mind for him for sure. It's possible that they realized earlier than we even imagine that he was not the answer, but that's all speculation of course.
I know that the addition of a couple of draft picks at S and CB, combined with a FA signing or 2 will likely shore up the secondary much more for next year, and allow DMcC to continue his role at FS. The growth of Dennard is certainly another factor as he'll head into his 2nd year, along with the FA moves of possibly retaining one or more of the Talib, Arrington, Chung combo.