When the season began I had high hopes for the defense. On the defensive line the Pats were going to have a rotation of some combination of Vince Wilfork, Ty Warren, Mike Wright, Marcus Stroud, Eric Moore, Kyle Love, Brandon Deaderick, Myron Pryor, Darryl Richard, Alex Silvestro, and then they added Albert Haynesworth, Andre Carter, Shaun Ellis and Mark Anderson.
At linebacker the Pats had what appeared to be a good group of improving young players: Jerod Mayo, Gary Guyton, Brandon Spikes, Jermaine Cunningham, Rob Ninkovich, Dane Fletcher and Jeff Tarpinian. The secondary appeared to be solid at both corner (Devin McCourty, Leigh Bodden, Kyle Arrington) and safety (Brandon Meriweather, James Sanders, Patrick Chung) plus rookie Ras-I Dowling.
It seemed to me that the defense had incredible depth, and all it would take would be for just a couple of these players to take the next step for them to become formidable.
Rather than specifically who surprised me, I would say what surprised me was that the defense was able to improve as much as they did over the course of the season when you consider the sheer number of how many of those players named above were lost, whether it be due to injury or lack of production, for either extended parts of the season or the entire season.
I think if I had to pick one, I might consider Vince Wilfork being able to not just be a three-down player, but being a productive three-down player. That's just unheard of for a player of his size.