People who treat their fandoms like a borderline religion and will start acting like Chris Crocker in his Leave Britney Alone video if you so much as lightly criticize the fictional things they enjoy.
There are far more offensive things people can do then insult something like Star Wars, Harry Potter, My Little Pony, Slender-Man, or anime. It really gets on my nerves when people get super angry and treat you like the Devil when you simply say in a respectful way,
"I don't like *insert name of popular show/movie/video game here," without any intention of being rude whatsoever. Let people have other opinions without shaming them for disagreeing with you please.