Michigan's defense: The DL
Where do I start? How about talent. There's lots of it. (for the sake of this discussion, we will assume Michigan runs a 4-3)
The DEs: LaMarr Woodley, Tim Jamison, Jeremy Van Alstyne, possibly some help from Pierre Woods. Woodley is a stud. He racked up 16 TFLs and 4 sacks last year and just looks dominant to me. He is way too quick for OTs to block and he's got plenty of strength to get the job done. I think he will have a monster year. Jamison and Van Alstyne on the other side both have good speed, though I'm not sure either has proven to be able to hold up against a rushing attack. There will definitely be some pressure on the QB from the ends, though.
The DTs: Gabe Watson and Pat Massey are the 2 main cogs on the inside. Watson is a mammoth 330+ lb force in the middle. He needs to work on his conditioning so he can dominate on every play and not just every so often. But he will dominate interior linemen. Massey is talented and has great height at 6'8" so he can tip some passes. Needs to work on playing low so that he doesn't get pushed out of the way on runs right at him.
The biggest improvements in the unit might be with the addition of new coach Steve Stripling. He's made a big impression already and many observers think he will help their talented group make some huge strides this year. If they can indeed step up their performance, it will greatly help the young secondary by not placing so much pressure on them. All in all, it is perhaps the most talented group in the Big Ten and one of the best in the country. Woodley and Watson are both potential first team All Americans this year and it would not surprise me to see them appearing on some pre-season All American teams.