Yes the last 3 years have been historically bad, but to say that the entirety of JT's 10 years we played outstanding defense is something of a stretch.
In my humble opinion the last TRULY dominant defense we fielded, first snap to last snap, was the 2002 defense. We had a great defense in 2003; without a few key mental lapses in Madison and against Michigan, that one could have been really great too. 2005 was also great minus a few moments. Over the latter half of Tressel's career, I think our defensive metrics were greatly skewed by the fact that we played atrocious offenses. I mean, really, really bad putrid ones. Now, for certain games, such as the Rose Bowl against Oregon and the 1st half of the Sugar Bowl against Arkansas, we had truly exceptional game plans. but, as a whole, no, none of those defenses in my mind were truly great.
I would love for nothing more than a defensive renaissance to happen this year in the 'Shoe, but, honestly, I think it's going to take more than one year and two new coaches to do so... its almost as if something is in these guy's heads, and frankly, it scares me.
EDIT: On a tangentially related note - every single time Stanford takes down Oregon and an ESPN commentator gives credit to David Shaw & staff for a singularly visionary gameplan against the Ducks, I see red and think of that Rose Bowl.