I think Boston fans are up there for being the worst, in part because of how quickly they will turn their allegiance away from a loser and how quickly they will adopt a winner and pretend they were there the whole time. Listen to WEEI and all of a sudden everyone on there is a Celtics expert, whereas a year ago if you tried to get Celts talk in you were mocked. Look at the Bruins making the playoffs this year and how quickly the bandwagon came a rolling in. Look at all the green shirts around (though Red Sox nation still clearly dominates) - every Beat LA shirt I see on someone makes me think, this person is just here for the ride and will get off at the next stop, whatever it may be. This group of Boston fans have become entitled spoiled brats.
Of course conversely are the oldtimers, the been through thick and thin, through the trenches. They don't appreciate anything today, because in their day the athletes were hungrier and played harder and didn't wear helmets and weren't paid millions of dollars, etc. etc. etc. I understand the history. I appreciate the history. But its the here and now, enjoy the ride, no need to be all grumpy.
One defining moment of my sportsfandom and disdain for the general Boston fandom came at the Fleetcenter maybe 6 or 7 years back. Its the Celtics vs someone (I went to a lot of games back then) - and Walker misses his first 3 badly, and this guy is just getting all over him, booing, yelling trade him etc. Walker goes on to have a terrific game, like a 20/10 leads a run towards a win, etc. The guy does a total 180, "OH I love Walker, he's the best, Walker! Walker!" - it was so obnoxious and telling....
PS - I like most of the fans on here, so this isn't aimed at anyone!