Didn't the defense finish in the top 30 in nearly every category last year? That's pretty good if not great. They might have been solidly great if they weren't on the field for entire games.
And in 2013, even with several epic beatdowns, they finished top 35 in total defense.
Nationally ranking aren't apples to apples comparisons. Schedules and quality of opponents vary too much. The best metric IMO is comparing your in conference stats vs your conference peers. Looking at just the Atlantic that means 6 like games. The 2 crossovers create some difference but there isn't a significant difference between any of those teams. Last season was statistically our best D in the last 6 years despite the putrid O.
Points per game we were 4th out of 7 teams in the Atlantic. This number can be skewed though because of the O.
Yards per game we were 4th. Again this can be hurt by having a bad O.
QB rating we were 6th. IMO this is the best number to judge your passing D. It takes into account completion %, yards, TDs, INTs, and in a way QB pressure. The more pressure you get the worse you would expect the rating to be. We were pretty bad in this regard.
Yards per carry we were 4th. IMO this is a better indicator of your rushing D as the pure rushing yards per game number can be skewed by pace of play and the score of the game, as teams ahead rush more. For instance GA Tech was 7th in ACC in YPG but 12th in YPC. You can argue that we would be better here if not for the bad O. Assuming that the D got tired.
Sacks we were 5th. I hope that this year's DL can do a better job.
TFL we were 6th. Again the DL needs to step up.
Turnovers we were in a three way tie for 3rd.
3rd down D we were 3rd which is pretty good. Especially if the D was "tired".
Redzone TD % we were 2nd. This is really good. It shows that we tightened up once teams got into the Redzone.
20+ yard runs (big play) we were 3rd which is good.
20+ yard pass plays (big play) we were excellent leading the Atlantic and the ACC.
The ACC Atlantic last year was pretty bad on O. BC, Clemson, Louisville, and Wake all had their struggles. That is going to skew national rankings. IMO it doesn't matter what defenses do in the MAC, B12, etc. What matters is SU's opponents. Last year we were decent vs the run. We were mediocre vs the pass. We were good at preventing big plays, getting off the field on 3rd down, and preventing TDs once in the Redzone. Overall I would say that statically we were a decent D. However taking into consideration our awful O I think that you can say that we had a good D last year.
Looking back at 2010-2013 the D underperformed vs last season statistically. IMO not a single one of those defenses were good. They were all decent. IMO a D can be great, good, decent, mediocre, bad, or awful. There is nothing wrong with being decent. It is actually a positive. However it seems our fans think of Shafer as a D genius and statistically there has been no supporting evidence.