Certainly the defense has changed over the years, but I would not be surprised by the overall results given Belichick's emphasis on defense over the years.
Do I suspect fans forgot about defense last year with the prolific offense? Certainly. Did defense take a hit losing to the Colts in the championship? Yes.
Since 2001, in my opinion, the defenses have been outstanding or not based on the health of the front 7. Given the complexity of the schemes, it is difficult to put a replacement in that group and get by with the secondary. The poor defensive years reflect injuries to that group and an inability to either stop the run or pressure the passer into making mistakes. While such a conclusion may be true of any defense, I believe it is more so for the Patriots.
Belichick had a good D in 2001. He built a great defense in 2003 and 2004. The defense has evolved over time, and Belichick has kept an eye out for new blood. But linebackers capable of filling the responsibilities of this defense are not a dime a dozen, so it is always interesting to see what happens with the new acquisitions. I like Thomas. Mayo has looked good so far. It will also be interesting to see if the D-line can be kept together in the long run.
Bottom line is I like the defense but I suspect with the Brady/Moss story last year the population may discount its contribution to victories. Do I think this is the 2003 Patriots defense capable of pounding opposing offenses en route to a championship? I hope so, but I am not sure. Good, but maybe not great. Whether it develops into a great defense with the new additions remains to be seen, but I like what I've seen so far.