In order:
Star Trek fans: If it's not cheesy 1960s-80s pseudo science, or has too much action, they hate it. They destroyed all hope for Enterprise, then whined about how awesome it was.
Nightwish fans: They hate any lead singer that's not Tarja... until they leave the band. Then the tune changes to how great they were.
Star Wars fans: Okay, I'm not debating that these people aren't understandably cautious about accepting anything new, but when something good finally does come along, they pick it apart.
Patriots fans: Loose a few games, blame the refs and whine about it for a week. Lose multiple games... leave at the end of the 3rd, break out the pitchforks and torches, and crucify Brady and BB!
Steelers fans: Bunch of bandwagoners living in a dream world. Ask them to name a SINGLE player on their starting lineup and they're like, "Duh... but I thought Ben Rotholisberger throw touchdown pass to Troy Polamalu!"
Thoughts? Did I miss anyone or have honorable mentions?