It's the pass rush. A fearsome pass rush will make your defense look worlds better.
You can't expect DBs to cover for 4 seconds in the modern NFL unless they are Pro bowlers...GB's aren't.
They don't have anyone in the secondary like Nick Collins who took iffy angles but had enough speed to erase mistakes, or the versatility of Woodson. Maybe Josh Jones can grow into Collins, but guys like Woodson -- DPOY caliber guys -- are rare.

You can't tell CM3 or Perry to take a pay cut - they are the only pass rushers they have. It's up to TT to spend a few high round picks on pass rush.
Given drafting is basically a coin flip, you have to take a lot of guesses to get it right. Trouble is, when you use high value picks you can't afford to guess wrong. Fackrell will stick because of where he was drafted and because he's decent on special teams. This is basically a redshirt year for Biegel and Montravious Adams. I'm pretty sure TT & Co. know what they need to do to the roster...but until then all we can do is watch.

The argument that most teams draft better on defense is iffy IMO. Drafting at the top of a round vs. the bottom can make a huge difference on roster talent, some years there are only 10-15 bonafide first round talents in the draft...and GM's make lots of mistakes. Look at the drafts the rest of the NFCN have had over the last 4 years...Green Bay has drafted at least as good as the rest, and I'd argue only Minnesota comes close to having drafted as well, mostly because they hit on several defensive players.