While I agree regarding pressure, there's a couple other things I saw on every one of these playoff defenses.
1) They swarm like madmen-they're all very active and quick defenses. Sure Alexander would break a couple runs against CHI, but even then there were 3 guys taking him down. More often than not, he was getting hit by 5 guys at the line of scrimmage and the tackles were always dogpiles. I'm not sure how else to describe it, but somehow Chicago's defense was always in the right spot at the right time on almost every play....and they were throwing hits.
2) They had a swagger. Every time they made a play, they were congratulating each other and making a scene. They wanted everyone to know who was the boss.
I just didn't see much of that all year from Dallas.