So true. Had some spare time and spent the weekend reading up on football. It's amazing the nuances coaches adopt. There are only so many schemes out there, but the fascinating part his how each coach adds his own wrinkled to them.
Marvin Lewis, for instance, when he came from Baltimore ran a base 4-3 over, with a lot of Cover 6 mixed in. He wanted big, two-gapping linemen who he would stunt often, allowing the action to get spilled to the linebackers who didn't have to be big, but they had to be active, sure tacklers. The unique part of his scheme was how he'd take multi-skilled players like Peter Boulware who was an outside linebacker and line them up as rush ends on passing downs. He tried to do the same with Lavarr Arrington in Washington and it looked like David Pollack (a college DE drafted as a SAM) was going to be that guy in Cincy. This clashed with Leslie Frazier's philosophy, his DC at the time, who came from the Buddy Ryan school.
Frazier ran an over front, but he wanted the lineman to be one-gapers and quickly penetrate up the field, pressuring the quarterback without a ton of read and react. Needless to say the relationship didn't last, and the Bengals haven't really pressured the quarterback the same way from the front four since.
Marvin Lewis himself came from Pittsburgh when LeBeau brought in all that zone blitzing madness from the 3-4, but largely went away from it as a coordinator. Other Lebeau disciples, Jim Haslett, and Dom Capers had mixed results in other stops largely due to personnel, but stayed aggressive -- Capers in Green Bay especially.
Some teams defy description. Palo's Pats have a defense rooted in that Chuck Fairbanks 3-4. Big players that just want to beat the hell out of you. Huge, two-gaping lineman, big linebackers that didn't have to be speedy, but strong enough to hold the point -- fairly straightforward coverages that kept everything in front of them. But ole' Bill added his eccentricities. I can't even begin to think of all the variations he has applied over the years, from moving smart, but not extremely talented players around like Mike Vrabel. He'd play off the line, then drop onto the line in rush packages. In their third Super Bowl against the Eagles he kept 5 linebackers on the field most of the day to combat Philly's speed. The last few years, he's employed a fair amount of 4-3 fronts.
I could literally go on and on, because there's so much cross pollination among NFL coaches. Atlanta's defense is almost identical philosophically to what the Ravens ran with Lewis, because Mike Smith was on that staff -- same thing with Jacksonville a few years ago when Smith was their DC, because Del Rio was also on the Raven staff. Del Rio was DC of the Panthers before he came to Jacksonville, and mixed the defensive package he learned in Baltimore with what John Fox did as the Giants' DC.
Rex Ryan was another Baltimore coach, but he works from a 3-4 that I won't even touch. He's got Bear fronts, he's got three safeties on the field, DB's blitzing from the stands, all kinds of madness.
Pete Carroll, Lovie Smith, and Leslie Frazier each have apprenticeships with Tampa 2 originators Tony Dungy and Monte Kiffin, but their defenses are quite different.
Carroll is a 4-3 under coach. One-gaping linemen. On the left side, you have a huge run stopping defensive end that looks like he should be in a 3-4. Red Bryant at around 300 lbs. filled that role most of last season. At right end, is the athletic pass rusher. And he likes to feature stud SAM linebackers in the Elephant position. They play on top of the TE, must be able to cover, strong enough to set the edge and skilled enough to rush. He's got Curry in Seattle, and had players like Cushing and Matthews to fill that role at USC. Cushing is about as perfect a 4-3 elephant as you'll find and I'm convinced that if he hadn't been suspended so many games last year, Frank Bush might have gotten another year as DC with his version of the 4-3 under.
Frazier works from an over front with 8 in the box. He'll mix in a lot of cover-2, but he's shown a tendancy to be a lot more aggressive with zone blitzing, creativity in moving his physical corner Antoine Winfield into the slot. The DB's don't have to be the fastest, but must to physical, sure tacklers because they are active in cleaning up in the run game.
Lovie is also more aggressive than the Tampa 2 would suggest, showing a lot of double A-gap heat, but he calmed down a lot last year when Rod Marinelli came on to call the defense and the addition of Julius Peppers. Same thinking with the DB's.
Most coaches can trace their lineage to one or two sources, but they tend to serve long apprenticeships which exposes them to a lot of disparate philosophies that they adopt into their own. You could write a book on what each coach runs and where he learned it.