They currently have a bunch of guys who are not tied specifically to any scheme. They've had straight nose tackles like wilfork, and have rotated in more penetration style players like Kyle Williams.
They tend to gameplan better than most team's at defense and are not dependent on scheme or deception. A guy might see 40 snaps one week and 15 the next, and he doesn't complain or say a word. Their defense currently isn't "great" - but they also don't have a bunch of high priced talent on that side of the ball. Good mix of young guys and older guys on cheaper 2nd/3rd contracts.
D-line - Flowers, Brown, Long, Sheard, Valentine
Lbs - Hightower, Roberts, McClellin, Van Noy
DBs - Butler, Ryan, and Jones - with McCourtey and Chung in the back.
No one there is like blowing you away - maybe hightower? But they liked what they saw in Butler and Roberts so much to jettison Revis and Collins. They got o-line help with a pick from trading away Chandler Jones. Now they have a good/great rotation of o-lineman after depth there previously was a weakness.
They also aren't afraid to dump players who want too much money - mankins, collins, and jones come to mind. The value they got back wasn't amazing necessarily, but they don't have majorly bad contracts on their payroll so that flexibility lets them be competitive with the mid-tier free agents that they covet. They can also add big short-term contracts because of their cap management.