As you already know, Michigan loses 4 defensive starters from their 2009 team. No big deal usually, but the coaches will also demote (Ezeh), move (Van Bergen), or demote AND move (Williams) at least 3 additional starters. We know that generally position switches are pretty bad news for defense and Michigan will have plenty of those among their expected starters. There are only 4 of 11 players returning to start at the position they started at last season (Martin, Roh, Mouton, and Kovacs). So, before we even get to the youth, depth, and talent issues with the defense, we have very little stability returning. Which gives us something in common with 2010 Alabama and 2007 Florida.
In previewing Alabama's D, Doc Sat calls attention to that Florida D...
in 2007, when the Gators had to replace nine starters from a nasty, veteran defense that finished in the top 10 nationally in every major category in 2006... The fledglings who replaced them that fall finished dead last in the SEC in pass defense and allowed at least 28 points six times
Even the most talented teams (and Florida's WAS extremely so, as "That same group went on to form the backbone of the Gator teams that ran up a 26-2 record with another BCS championship over the last two years") can't handle new starters...even with coaching stability and "another year in the system".
So while Michigan isn't facing quite the same attrition (we weren't exactly starting with a top 10 D anyway) we also don't have the same level of talent. IMO, Doc Sat's conclusion about Alabama applies to Michigan as well...
This edition of the defense may turn out to be very good in the long run, but if it has to be as good as the 2009 edition... you may as well be asking them to walk on water.
With all the inexperience, we Michigan fans shouldn't get our hopes up about the 2010 defense, but Florida tells us that having talent and a young starting lineup can yield some nice long-term results.