The thing that always impressed me about Bill Belichick is his ability to put together a competitive defense year in and year out. There was one year I was convinced their defense would be terrible because they lost so many good players and had a bunch of no names in the secondary. And then they were pretty good. That's the year I was finally convinced that he was an amazing coach (I don't even remember which year...)
He isn't an amazing GM and his drafts are average, but he's a great defensive coach, and at least he knows how to get the most out of his players. I just think the tank is completely dry on offense.
Brady taking a pay cut for all those years really helped them out too.