We had good defense last year. We had good offense. Statistically we were about middle of the pack everywhere. Like I said, it pretty much boiled down to having trouble running the ball and committing turnovers being the difference in every loss (except arguably ISU that I can think of off hand).
People try to over complicate things by blaming scheme, pass rushing, blitzing, lack of offensive luster...doesn't matter when you can't do the basics, run the ball well and not turn the ball over. That is consistent regardless of scheme. The same will be true this year.
As far as defensively, we need to be strong up the middle. I don't get overly concerned with passing yards given up. You need to be able to stop the run up the middle first. Eventually, teams that get inside the red zone have the field shrink on them and that's when the battle in the trenches is magnified. You have to win there to score on offense or prevent scores on defense.
We made simple, fundamental errors last season in the games we lost. Committed turnovers, weren't able to move the ball on offense between the tackles and that's the bulk of the season. It's really pretty simple when you boil it down.