I rarely get annoyed with fandoms enough to actually leave it, but there are some things in fandoms, and about fandoms, where head-scratching is required.
1. Anti stuff: I think of the word Anti as a sort of negative thing, and kinda childish. For example, in the fandom Soul Eater, some people are Anti-KiddxChrona, or Anti-KiddxLiz or Anti-Black*StarxKidd or something along those lines. I, for one, think this whole business being Anti-something thing is stupid. Obsessively hating something, like the Twilight series, is just as bad as obsessively liking something. I have similar feelings about people calling themselves 'Insane'. I used to do that too, but now I see that claiming you are 'sane' is a much better option.
I am not Anti-anything, since it seems childish to proclaim to the world that you will forever dislike something. I certainly am not against someone being pro-something, or anti-something in real life, or even disliking a pairing, but fandom is about fun, and proclaiming you are anti-something just starts wars that ruins everyone's fun. Common sense should tell you that if you dislike something, stay away from it. If you spend your time obsessively hating something, then you get no real productivity from it.
2. Obsessive fans: This is where you wonder if these people need profesional help. I see a lot of obsessive fans in a variety of fandoms that make me facepalm. An example would be yaoi fangirls, who will attack anyone who says their yaoi OTP isn't canon. I've seen them in the Naruto fandom and the Durarara fandom. It's so weird. I like slash, and happy to be one of the coherent and mature fans of such. Another example is the Puella Magi Madoka Magica fandom, where the fans are rabid and they are ignorant, and where they attack anyone who dares criticize the anime. It's pitying, and disturbing. I don't even know why these people do it. I will never understand those people cause I have no desire to be like them. At all.
3. CCxOC: There is waaaaay too much of this in the FFN archives and on Deviantart that I just want to puke. It's annoying, it's no different than Twilight and it makes it harder to find the diamonds in the rough of fanfiction and art. I feel like it's just cluttering every fandom and making my browsing more difficult. I consider these kinds of people on the lowest part of the geek hierarchy. However, people who create OC's for their OWN universe and storyline only I consider on the higher level, since they are being creative and productive and telling a story. It also kinda goes for people who create an OC for a series yet don't pair them up with a CC.
4. The Overdone Alice in Wonderland Theme: I get tired of seeing it in fandoms, in fanarts, in fanfics, ect. It just bores me to death and grates so much. In some cases, it's pretty neat, but for the most part, poorly done and terribly done at that. I'm sick of seeing it.
I'll add to the list if I think of anything else. Right now, I need sleep. Night all!