Losing Dumerville hurt. He and Von Miller were deadly at times during the Tebow year.
They picked up Pot Roast Knighton last year off of the Jags and he's been a nice addition.
They had DRC last year and basically replaced him with Talib this year, both guys on 1 year deals. They signed TJ Ward and DeMarcus Ware this year, too. A lot of guys commented on Ware's leadership, but wore down at the end of the year and was just not as effective. Talib's been pretty good against lesser opponents but he got run around by Hilton. Part of that is on JDR for not moving Chris Harris over, maybe. TJ Ward has been pretty good but he got lost at times also.
You're probably right that the signings worked out pretty well, although I think their vision was a smothering defense like the Seahawks but they got one that was a slight upgrade over last year (8 v 9 in the FO rankings, though DVOA was ~3% better). It still couldn't get a stop when they needed a stop. I still think that the type of defense they were envisioning takes a few years to build just due to the communication involved and the necessary esprit de corps.