Not to be a buzz kill....
But I followed NEs stat line/rank on all the sites through last year (and every year). The Patriots were consistently ranked in the late 20-30s in yards and early 20s in points allowed throught the entire year....right on up till those two games against the Jets and Bears were the opponents just totally imploded. Then our defensive rank skyrocketed. Then had two games against MIA and BUF who had nothing to play for (and played like it).
So, I really don't think it's a 1st half vs 2nd half deal. It's more like the impact two total opposing meltdowns can have on a parity driven league. Cause once you get beyond the truly elite - the difference between the bottom end of the mediocre defenses and the top end really isn't that great. In fact, it's so small a pair of games can make a total difference.
And yeah, the 2010 D was mediocre. In fact, that might be a compliment.