The addition of Lawrence, Simmons, and Brock, the move of Sherels and Campbell, and the return to form of Van De Steeg had a lot to do with the improvement on defense. That is like getting 6 new starters. The focus on creating turnovers was also paramount to the improvement because it helped get the D off the field and it helped keep the scoring down (the rank of scoring is much better than the rank of yardage allowed). The schemes from Roof also played in to improving the pass rush and better defending the pass.
We still have a long way to go in terms of improving the starting talent on our defense. I won't call out any particular players but I can think of one starter that actually has the talent and athletic ability to be a D-2 player but has been overachieving to see the field at this level. When the 2008 and 2009 recruiting classes get to be juniors and seniors we will actually have the talent on D for the Big Ten conference but it's not fair to excpect very many players from those classes to make an impact and start in 2008 or 2009.