Jonathon Chait writes a great defensive autopsy article at Rivals. This is not premium content, so the link should work.
His analysis has 3 main points:
1) The thin D-Line created problems, with guys too small being forced to play a hybrid nickel, and getting man-handled by bigger O-lines.
2) The linebacking corps was atrocious, and this was NOT caused by a lack of 4-stars on the roster.
3) The thin secondary caused many problems. Perhaps the worst was the emergency move of Woolfolk from safety to corner. When Woolfolk was moved back (for OSU,) the defense actually did reasonably well.
Nice to read a reasoned analysis, with some hope for the future. On this last, Chait is going to follow up with a 2nd column.