I think the defense is playing really well this year, and I suppose that's not a surprising statement - or wouldn't be at the beginning of the year. What is surprising is that this isn't due to the front 7, but rather the secondary.
In 2011 -- our defense was at it's peak, with the front 7 dominating (1 or 2 rushing TDs all year, no 100 yd rushers for many many games in a row, etc.). The Smith brothers terrorized the passers, and the twins in the middle of the field dominated the run. Justin Smith was the best player in the NFL, Aldon was the most exciting defender, and McDonald and Brooks were more than holding their own--they were dominating quite often as well. The secondary benefited greatly and resulted in career seasons for Goldson, Whitner and Rogers. Brown continued his solid play and rookie Chris Culliver flashed (huge game against CJ).
In 2012 -- the defense was still just as fierce, but as the season wore on, obvious holes were noticed in the secondary. Whitner's play dropped of, Rogers regressed back to solid but not great, and Culliver didn't develop quite to the level that we hoped. Goldson made splash plays but got exposed far too many times. Brown continued his solid play.
By the time the 2013 Playoffs rolled around, our secondary was getting torched regularly, and the weakness was discovered in our defense. It was already the weakest link.
In the 2013 Offseason we lost Goldson to FA, Culliver to season ending injury. We failed to use any early picks on a CB, and traded up for a FS who very few were excited about, and no one had pegged as a anything but a downgrade from Goldson.
The outlook for the 2013 secondary was weak. The start of the season didn't disappoint. Nnamdi got regularly beat, with Rodgers, Wilson and Luck picking him apart. The rest of the group did not hold up much better. Well, Brown continued his solid play. Perish Cox was looking to be a liability for the second year in a row. But then the secondary started to Gel. Knocking down passes, picking the ball off, returning turnovers deep (or all the way into) the opponents territory. Brock is the obvious surprise, along with Reid who is playing at an unbelievable level and is a major upgrade so far. Whitner got hit with a few bad calls early, but ever since he's continued to have the best season of his career. Brown is continuing his solid play, while Cox has been mostly quite. Currently Rogers is the weakest link. This secondary is covering, creating turnovers, stripping the ball, hitting hard and picking up the slack while the DL deals with injuries, the LB group waits for Aldon to return and Willis to heal, and offense is trying to find it's stride.
With all the things working against this secondary, for this to be the group helping us win right now, is the biggest story from the first 7 games for me.