There’s a lot of things going on with Seattle’s defense. I’ll try to be brief.
I think the key elements are their consistent pass rush up front and Earl Thomas. With those two pieces in place everything else looks even better.
They are a Cover-3 team and Thomas’ range and anticipation allow them to have Chancellor in the box destroying people. There simply aren’t many free safeties as good as Thomas, even McCourty. He makes very good players look great.
The physicality and disciplined tackling are quite impressive considering the rule changes. Credit to Pete Carroll there, it’s certainly not easy.
But it shows that when you have a versatile and athletic front seven that can rotate and cause consistent chaos, it really makes a defense.
What you have to admire about Seattle’s defense is they just do what they do and ask opponents to beat it without disguise or tricks. I think the Patriots emulated that somewhat this offseason with the concerted effort to bring more physicality to their secondary, and a more diverse range of options in their front seven.
The Patriots will never be a straightforward Cover-3 team, but hopefully with McCourty and Revis/Browner, they can replicate some of what Seattle does. The Seahawks had great success against Peyton Manning, so it’s obvious why the Pats would try to take a page from their book.