A couple people have touched on this, but one thing to keep in mind is that we are only two games into a new defense, with new coaches, and a new scheme... not to mention the fact that Dunta, Cushing missed the entire preseason. Antonio Smith and Busing weren't on the roster last year. Add to that the reliance on a number of rookies: Cushing, BMcCain, G. Quin, Barwin... and the general youth and inexperience on the defense: Adibi, Okam, Okoye, Robinson, Diles, Barber, etc... Most of these guys have only been in one scheme as a pro. I think it's natural that it takes some time to adapt.
Back to my original point when I started this thread, as bad as this defense has been, clearly there are signs that its play can improve a lot... and quickly. My guess is that in two weeks we will be 3-1 and the discussion will be whether the defense can look good against a real offense (Arizona) or if they just looked good because we were playing Jacksonville and Oakland. Anyone want to wager on that?