I agree about our defense pulling us through some games, however, I must tweak your argument about our offense. While I do think we took a step back offensively, it was really only in two areas: the WR position and quarterback position. Here are the positives of our situation:
1) Zac Lee is physically a better QB then Joe Ganz. He has more arm strength, speed, and has taken a JUCO team to a national championship. Granted, it's JUCO, but still.
2) Helu and Castille are arguably two of the best backs in the Big 12. Their numbers don't show it considering they had to split time with Lucky last year, but they are both very talented. Castille sliced and a diced the best defense Nebraska had seen all year in Clemson for 125 yards. And Helu put up 800+ yards, more than half of which came in the final four games.
3) We lost two OL starters, but the entire left side of our line (center, guard, and tackle) are returning starters.
This year, expect Nebraska to revisit the days of old a little bit by running the ball more than they did in 2008. Probably won't see a lot of the option or the Oz-bone, but you will see more running plays in order to help spur our passing game.