Say game "X" comes out and has sequels. The sequels have improved visuals, improved controls and mechanics, but has only minor changes to the base story and concept.
Now say game "Y" is released and has sequels. The sequels have improved visuals, improved controls and mechanics, but has only minor changes to the base story and concept.
Now fan "A" loves game "X" to death and thinks of the sequels as vast improvements over the previous versions (sans a couple of terrible mistakes the developer made that made it too different of a game).
Now fan "A" takes a look at game "Y." Game "Y" is of another genre then game "X." So fan "A" clamors that it's the same game over and over again and that anyone that enjoys it has been fooled by the evil corporation that developed it. If anyone tries to defend it, then they must surly be a close-minded "One Brand to End all Brands" fan.
This is the fan that usually won't be happy until game "Y" is changed enough to be exactly like game "X."
This is the type of fan that, if this was food instead of games, would bash Waffles for being boring and unoriginal until they add enough changes to make it in to a hamburger.
It's the "All must play as One" Fan.
That is but one example of fans I hate dislike. Other Examples are:
"If it was good once it'll be good again" Fans: The Sequel/Remake pushers. (Just the really pushy ones.)
"The Reader trumps the Writer" Fans who think they know more about the game and it's characters then the developers. (That's like knowing more about Luke Skywalker then George Lucas does.)
"All must look like One" Fans that think a game is bad because of it's graphics or choice of visual style. (Look, if you don't like the cartoony style of Wind Waker that's fine, doesn't make it any less of a great game though.)
And while we're at it, let's add in the "All must sound like One" fan, the rare ones who think that a game sucks because the character doesn't act like their favorite character does or the story is told differently, to complete the Trinity that is "All must be like One" Fan.
"I just like to Complain" Fans, that will complain about every last little thing they can because they don't understand that the concept of nothing's perfect applies to games as well as life. This coupled with the "I'm no developer, but you should do this" fan who's ideas are just bias and not thought out. A bias idea would be suggesting they add Cloud from FFVII into Super Smash Bros. simply because it was your favorite game or character. A non bias idea would be to add a generic Black Mage, since they're common in the FF world and have appeared in actual Nintendo games.
And because this list is getting long, I'll end with this:
"Sonic Fans"
I'm a Sonic Fan, we're a delusional over-demanding lot aren't we? We're so hopeful the next game will be good, yet we're so jaded by how bad his games are that we can't let go of the past. I'm willing to bet that every fan type listed on this topic can be equated to some Sonic Fan.
Sonic Team really needs to take a break.
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EDIT: While we're on the subject, I'd like to give a special hate filled shout out to "It was a Game?!" and "The Original should Mimic the Copy" Fans. Those special people who became fans of the shows based off a game and, upon finding out about the games, wanted the games to be more like the show or constantly refer to the show as if it's the Original incarnation of the Series.