Aside from maybe an improvement at DT next to Wilfork (and I'm not that low on Kyle Love either), the front seven is fine. I would like to eventually see a move back to the 3-4, but the front seven itself is stout. As you say, the secondary is a mess...
1. McCourty is awfully inconsistent and struggles in man coverage.
2. Arrington is one of the worst corners in the league.
3. Dowling is injury prone.
4. Dennard is a promising rookie.
5. Sterling Moore gets burned a lot and struggles with receivers he can't physically impose his will on.
6. Patrick Chung is bad against coverage and solid against the run. His skills don't exactly scream Pro Bowler in today's NFL.
7. Steve Gregory is just north of a JAG, has had injury issues throughout his time in the NFL, and was usually caught out of position with the Chargers.
That's six out of seven guys that regularly see playing time in the starting units that can either be replaced or improved upon. To expand on my position, many of the same compaints were made about Pees a few years back. One of the greatest defensive minds in the history of the game took over for him the next season. The results were largely the same. That same mind was the DC last year... he oversaw the second worst pass defense of all time. Again, it's not a coaching issue. It's a talent issue.