I hate them because almost every story I come across has at least one of these four things.
1.Slash. The author takes two characters of the same gender, and for no reason they fall in love. All that ever happens in these stories is the characters having a awkward, out-of-character conversation, followed by kissing. I hate trying to write reviews for them because half of the time, I get called intolerant for not loving their slash story
2. Out of character. All the time, I see fanfictions where the characters are completly different from the character in the original story. For example, I've saw a fan fiction not too long ago, where a group of runaway children who are just trying to survive, start a band, and everyone turns into and emo. I hate, HATE these stories.
3. Crossovers. I see this one all the time too. Authors try mixing two different books together, which is fine, but it usually ends up completely out of character.
4. Off the wall, bizarre stories. I don't know what it is with fanfictions, but people come up with the strangest stuff. I actually found a story where according to the description, a boy and his sister falls in love, and then the description said something like "Warning: Hardcore Lemon. Don't read unless you are at least 12 years old".