As someone who attends games (and playoff games) regularly at Gillette, I don't think it is as bad as people make it out to be, particularly in the playoffs. And since it doesn't seem to impact performance I don't worry about it.
But if you are looking for reasons why Gillette isn't as loud as other places, the biggest factor by far is that the Pats win all the time and people just don't get jazzed up for regular season games anymore after 16 years of winning almost every regular season game they play at home. It's the most first world of all possible first world problems to have. The Pats must not only win, but kill their opponents, and be in the Super Bowl every year or fans are disappointed (this message board is hardly any different in terms of how some fans react to not only the losses, but even the close wins). They are never the underdog and are always favored. Given that formula, people just aren't going to get as excited about most regular season games as other places.
I don't buy the argument that the Krafts have priced the "real' fans out. First, that is true of every other new stadium that's been built. They all have club sections in the middle level with super expensive seats that regular/normal folks cannot afford. So why would this impact New England more than Denver or Seattle or wherever? Second, as someone who has been going to Pats games regularly since 1997, the only thing that's changed about the game experience is fewer fans who are passed out in their own vomit or piss, or being belligerent and picking fights. I don't think the super-drunks were ever particularly loud before, but I can live without their presence at the games even if it means less noise.
The Optum club stuff was bullshit on the part of the Krafts, but the crowd noise complaints have been around much longer than that club has existed.
Lastly, the configuration and capacity of Gillette really matters. Not only does the open end suck, but the 300 level just feels significantly farther away from the action than other upper-decks in stadiums I have been to. In the games I have gone to up in that level I have found it hard to expend the energy to make a ton of noise because it seems pointless from up there.