Seattle's defense was built by drafting extraordinarily well in the secondary and for linebackers and spending in free agency for defensive linemen using money that's available when your team is only spending $800k on a quarterback.
The Patriots had two stud defensive ends who were completely shot at the end of the year because they played 95% of the defensive snaps and no backups. Their interior linemen - post injuries - were merely fine.
The Seahawks, meanwhile, featured four edge rushers in Avril, Irvin, Bennett, and Clemons - three of whom were signed to post-rookie deals, and one of whom was a 1st round pick - who could have started on any team in the league, and three interior players in Mebane, Bryant, and McDaniel - all of whom were post-rookie contract - who could have done likewise. This rotation where none of the guys play more than half the snaps kept everyone fresh for the entire year.
Not sure the secondary success can be replicated except by blind luck, though.