Related to writing such as stories, characters, etc.
This is not intended to discourage people from writing; rather, it is so you know what to avoid in the future. Or, you could just rant on. :U
As a reminder, pet peeves are personal so not everyone may necessarily share them. :D But that does not mean that you can flame!
I vaguely remember a girl on LJ who made a lot of rants on fantasy which helped a lot of people improve their writing. :) Thus, this thread.
Anddd... well, my pet peeves?
I dislike reading huge blocks or walls of text. D: They make it difficult for me to enjoy the story.
I'll think of some more later. C:
That's probably mine as well. Just so much text, when the paragraphs could be broken down a bit more.
Another thing I'm finding to be annoying is when I read a section where the character is supposed to be thinking, but the thoughts are not in ' ' so it's confusing to know when the character is thinking or when it's just text in the story.
I don't like having author's notes slap bang in the middle of text, interrupts the flow of the story for me and makes it less believable.
If anyone else feels the same, avoid The French Lieutenant's Woman like the plague - just a thought :)

I hate huge walls of text as well. There's absolutely no excuse for it and paragraphs are your friends.
I really hate excessive use of commas as well: either break it down into smaller sentences or learn how to use colons and semi-colons (and learn to use them properly, too).
Sort of linking in with walls of text, something else that bugs me is when people info-dump. This can include descriptions of characters in perfect detail, to over the top detail of scenery or telling us everything important that's happened before the actual story starts. It's often much better to drop your character in the middle of the action, hook the audience, and slip little pieces of information in along the way if you really need to. Nothing turns me away from a story more than having a bunch of information dropped in front of me. It ruins the flow.
Same for me. I hate reading a huge block of text.
Cleared - Oversized
As much as I dislike HUMONGOUS text blobs, I also dislike when the story is mainly dialogue. Lol I'm so picky with reading, I also can't stand repetition.
ex.
"Hey there." the blonde girl, Nina says. "Hi Nina!" the brunette boy Jake says "What's up?" Nina asks. "Nothing." Jake says.
._.
Ahh, the thoughts could also be italicized; I've seen that in fanfictions. A/Ns (Author Notes) in the middle of the stories annoy me too. :/ The best way to put them would be at the top and/or the bottom but never more than two notes. :D
Heh, I'm a little guilty of too many commas ^_^; I'm trying to use less. Well, not too many, commas, like, so. XD; I also have a little problem with the dialogue since it's used a lot in my stories. :)
I dislike Mary Sues or Gary Stus too. :P If you don't know who they are, they break the rules of canon and are really unrealistic. IE, Character is so beautiful that everyone falls for her on first sight and she can translate 56 languages and she gets A+ in all classes and she works at 5 places and she makes beautiful pieces of art in one hour (yes, even 100). That's just unrealistic. xP
There are some cliches that I dislike. :/ Love triangles, unless executed very, very, very, very, very, and I mean very well. It's just... there.
And lol, I've read a couple of Christie's works. They were pretty neat. xD
I'm guilty for long paragraphs when I actually do need to explain something in greater detail.
Pet peeves...
;
^ If you don't know how to use this little fellow, then DON'T. It drives me nuts when authors throw them in with NO idea how the hell they're using them. It's just... ugh!
Also, I hate cliched phrases. Things like, 'Her eyes could see through right through my soul', or 'it was black as night'.
Characters who have impossible names to pronounce.
Massive blocks of dialogue.
Antagonists who turn all nice and fluffy in the end.
omg this this this this this
I have so many pet peeves but most of them are character related.

I just don't like super-long descriptions. I think it's called purple prose or something? heard the term on FFN
:|
Excessive angst. It's been done to the DEATH. ;o;
Also, most poetry these days. :P.
I can't stand it when someone constantly uses "the other" when they write or roleplay, especially if it's without specifying the other WHAT.
"He glanced at the other and shrugged."
I have met SO MANY PEOPLE who use this phrase over and over and over. But not "The other man" or "the other soldier", no. Just "the other."
I could just scream, agh. x:
A whole bunch of things...
And quite a few other things I don't feel like boring you with. ^.^;;
I hate when I sound pretentious. I couldn't care less about other writers for some reason. Yeah, if they sound overly flambuoyant, I will probably get annoyed, but for some reason, it is harder to accept myself when I come off that way. I don't like getting purple, I like clear, concise prose as much as the next person, with just enough pretty descriptions, but it just sometimes feel like I can't resist being bombastic. Sometimes I just want to stick a note to everything I write saying, "Sorry if I sound full of myself. It really wasn't my intention."
...or whatever. I don't know, it just really bothers me.
...
Present, active tense. As in:
"She stands by the window and chops up the vegetables for dinner." vs. "She stood by the window and chopped up the vegetables for dinner."
...What an odd sentence. xD Anyway. Both of them are correct, technically, but I just prefer the second one much more because it's used much more frequently in the stories I read. The first one just seems so much more... choppy, I suppose.
And when a single world is used extensively, like in a conversation, everyone "said" something. My gosh, I feel so bad for the word "said", it is terribly abused. D8
A lot of my pet peeves are listed here already, but another one of mine is overuse of the same adjective.
When I was roleplaying with a certain member, she liked to use the same adjective with every single action her character made. Let's pretend the word was warmly (it wasn't really, though).
The captain smiled warmly at the stow away.
Next post.
He warmly laughed, and walked away.
Next post.
Captain Smith stared warmly at the girl's pretty face.
That particular person I roleplayed with also used smiled way too much. Her character never grinned, grimaced, smirked, or beamed. He/she only smiled. It positively drove me insane!