My pet peeves:
Male pregnancy: I have nothing against writing about a Male/male relationship, but usually if a character who is 100% male, gotten pregnant with their "significant other's" child. I hit the back button and remind myself never to read that story again.
Mary-Sues and Gary-Stus/OC: Hate it when an author decide that they want to write themselves in to a story to get the character they want. Their OC are usually beautiful, smart, perfect, admired by everyone, gets the love and attention of their favorite character, gets into a battle-killed by enemy-mourned by everyone including love-comes back to life through prayer-reunite with love-married-lived happily ever after. I want to throw up.
Passing one giant paragraph as their story
Bad grammar, spelling mistakes and short cut writing: No one's perfect and usually there are mistakes made during writing and that's ok, but if they simple words like 'you' to 'u' and they make the same mistakes in grammar or speeling for over 10 chapters, the author is just being lazy and it shows he or she doesn't care about what they write and is putting a story up so that people could acknowledge them on the internet.
Changing a character's personality or blowing a trait way out of proportion: Ok, people know that Naruto isn't exactly the sharpest tool in the tool shed, but he isn't an idiot that wouldn't know up from down. Some stories constantly have Naruto insulted by others, like people calling a dumbass or having the word idiot thrown at him millions of time in a single chapter. Then the author makes Naruto do something very stupid afterwards which makes the reader think that he deserved to be called names, making them focus solely on Naruto's stupidity and lose focus on his good traits. And then there are authors who have Naruto insulted and makes him emo like, making him cut his wrists or drown in alcohol.
There's a bunch of others, but they are pointed out already in here , so I'm just leaving down things I hate in fanfiction writing.
From the flight deck before takeoff: I'm only gonna say this once: You have to turn off your laptop, iPod, Game Boy, CD player, BlackBerry, blueberry, strawberry, cherry, and Halle Berry -- yes, you have to turn her off, too! You have to turn off anything that isn't keeping you alive.
--JetBlue flight, JFK