I think there are a number of aspects:
1. The previous regime totally failed in terms of implementing a defensive system. They had no idea what they were doing, which made even our decent players (DJ, Champ, Doom, Peterson) look like crap. As McD said in one of his press conferences, there wasn't even the basic foundation of a 4-3 in place to build on... hence why they went directly to the 3-4.
2. The addition of Renaldo Hill, Brian Dawkins, Andra Davis, and hell, even Vonnie Holliday has greatly increased the leadership on the defensive side of the ball. That was something we sorely lacked before.
3. Discipline has been a big focus on defense. Players are sticking with their assignments. The freelancing ways of the past are gone (along with some of the worst offenders, e.g. Jamie Winborn and Nate Webster).
4. The current scheme is designed around making the most of player strengths. Find out what a guy is good at doing and put him in a situation where he can do it. We don't have the most glamorous defensive roster, but the guys we have play hard, know their assignments and have the necessary skills/abilities to execute them.