I've mentioned this before (even in the game thread this week) but I honestly think that Belichick as a coach is overrated. I think he lucked into Brady, perhaps the greatest QB who's played the game professionally, and too many people just assumed that he was the driving cause of the success that Brady has had.
I'll point to two bits of evidence on why I think Belichick is overrated... neither of which is a smoking gun, but still substantial enough that I think it's telling:
1. Record. Over all, including playoffs, Belichick as a HC has a pretty nice record: 328 wins to 163 losses, or a 66.8 winning percentage. Not bad at all! But most of his HC career has overlapped having Brady as his QB. In games where Brady was NOT the starter for Belichick, his record is uninspiring: 79/87... or 47.6% The reason this isn't a smoking gun is that the sample size here is small and consists mainly of his time in Cleveland and a few years in NE w/o Brady as the team has struggled to find a starting QB. However, if Belichick is to be considered one of the best coaches of all time, should he not be able to coach up a replacement QB? Should he not have been able to find some level of success outside of his once-in-a-generation QB?
2. Coaching Tree. I would expect that a coach who is to being considered as the greatest of all time would have at least a decent level of successful coaches developed under him. Even if he wasn't actively trying to train/teach his lower level coaches, I would expect some level of his thinking/talent/ability would wear off on his coordinators. And yet I can't think of a single person, outside of Nick Saban, who's had even a solid, successful career as a HC after parting ways with Belichick. And even looking at Saban, he was Belichick's DC during his Cleveland years when Belichick only had won winning season.
We'll see if Belichick proves me wrong later, but as of right now I think Belichick is an average coach at best... I think he lucked into a super star QB and road on Brady's coat tails to fame and fortune.