My issue, at least lately, with Boston fans is the issue that fans of sports franchises who win a lot have: everyone seems to have an opinion but they aren't terribly knowledgeable about the game they have an opinion about. In some ways they're like the opposite of Seattle sports fans, who right now anyway are really, really lukewarm about any of their teams (Boston, to its credit or detriment depending on how you feel about this, isn't terribly "fair-weather"; even in bad years, the Red Sox for instance draw a good number of fans) but those who are are really nerdy and analytical. When I think of Boston fans, I think of Bill Simmons, who is funny, don't get me wrong. When I think of Seattle, I think of USSMariner and Fangraphs and the SBNation sites. When the Sonics were in town there was a pretty good-sized stathead community for that as well; for instance the single issue of Basketball Prospectus that was produced was done by a Seattle-area guy (Dean Oliver).
The other thing which used to drive me crazy about Sawx fans in particular was the whole Curse thing. There never was a curse, only Bostonian racism. Sorry, but I have zero sympathy for a club which went out of its way to find a bad review of Willie Mays so they could get away with not signing him. To his credit, I don't think Ted Williams was party to that crap at all, but then again Ted Williams was reviled by the Boston media during his time there.
I guess this is the same kind of bad that lots of other cities have. It just seems to be worse in Boston than elsewhere. West Coast fans can be annoying in our own way too, but that is generally the fair-weather-ness and the showing up late and leaving early to avoid traffic things, which are maddening but at least they don't make you get into fights with people.