Pretty much nothing. Lol.
While he was a great coach and was close to a lot of players...look at the history of defensive coordinators leaving the Ravens. Jack DelRio, Marvis Lewis, Rex Ryan, Greg Mattison..etc. Has our defense ever stunk? Not really we have historically been top 15 every year.
My point is that we still have our huge play makers/ leaders on defense. Which means were in good shape! Even if we lose Jarrett Johnson and/or McClain to free agency, we still have one the the BEST front offices in football. Do you really think they are just gonna let the Ravens have vacant holes? Nope. In Ozzie we trust. Look what he did last year with releasing Derrick Mason, Todd Heap, Kelly Gregg, Willis McGahee, and McClain. We didn't miss a beat and in fact I think we improved.
Same can be said with our next defensive coordinator, if they do bring in a new coach they will make sure he does it the Raven way. Don't fret Ravens fans, our organization doesn't have to rebuild every year, they reload.