The supporting cast and system.
Easiest example: Adalius Thomas. Looked good in Bal because he was playing with a bunch of great players on D, comes to NE and is meh.
Deion Branch goes from a Super Bowl MVP while playing with an All-Pro QB to a nobody in SEA.
Good player there doesn't necessarily mean a good player here.
Someone that I have some reservations about is Dashon Goldson. Before Harbaugh comes in, he's just a decent S who has to settle for a one year deal. After playing with Harbaugh, he's an All-Pro. I think Aldon Smith's arrival, along with Bowman's development, really helped Goldson as he plays behind one of the best, if not the best, front 7 in all of football. Think about it, Whitner, Rogers, and Goldson were all mediocre players before playing in SF. By 2012, they'll all been elected to at least one probowl.
I'm not saying Goldson is bad, because I've watched him play, and he looks pretty good. But if he came to NE, would he be the same player he's become in SF? He won't have that dominant front 7 to alleviate secondary pressure.
Just something to consider when people just start throwing out random FA's that they want BB to sign. Nnamdi goes from an All-Pro to All-Arrington by signing with PHI. FA's have to fit with the system and the group of players. They also have to be self-motivated...too many grow content after getting a big contract.
It's a risky business and am surprised so many people want the Pats to just starting signing these big name FA's left and right. Other than resigning your own players, I would say most big ticket FA's aren't worth the money spent on them