Since I've accumulated several over the years, here is a list of the things I hate when I read fanfics.
1) Sugar and caffeine highs are not good excuses to post bad fics!
Sometimes, we have those nights.
"Hey, Franzie, I got some Pixie Sticks and Coffee!"
"Awesome, let's have some."
30 minutes later...
"HEY, FRANZIE, LET'S WRITE A FANFIC!!"
"OMG YES IT WILL BE SO RANDOM LOLOLOLOL!"
The next morning...
"Woah, why did I write Goku having sex with an elk-man?"
"I dunno, let's put it on the internet."
Yeah, just because you wrote something while high on sugar doesn't automatically make it hilarious and "random". It's often obnoxious as hell. Don't do this.
2) If you want to put a mental disorder in a fanfic, do the research!
Mental disorders in fiction can create great drama. However, people can get very, very annoyed when you completely butcher a mental disorder. Schizophrenia does not make you yell "CHEESE MONEY NINJA!!!" every three seconds. People with multiple personalities don't interact with those personalities and have no memory of doing things that their other personality has done. Having depression doesn't mean you instantly streak on the black eyeliner and start cutting yourself.
If you want to portray a mental disorder, make it accurate. I can understand using them for dramatic purposes. But, I can't stress this enough, make it believable and accurate!
3) NO CUTTING. EVER.
Again, this can't be stressed enough. Cutting in fanfics has been done to death. It's not dramatic. It's not a good way to get people to feel sorry for your character. Barely anyone likes cutting in fanfics. In fact, this was one of the biggest tips in the fanfic panel at Otakon 2008 (for those wondering, I was the one who said the lame Sailor Moon overused plot. Forgive me, I was being impulsive).
4) Keep your facts straight!
If you're not sure about a canon detail, like, say, how to spell a character's name, refer to the original work or a trusted source (like a good fansite or, hell, Wikipedia). Constantly getting details from canon wrong or constantly misspelling characters' names makes you look lazy and that you don't care about the canon work at all. If there are alternate spellings, point it out! Alternate universe fics can be good things. Well, there's one exception, which is...
5) No high school fics!
High school fics, like cutting, have been done to death. Really, how many times have we seen Cloud and Tifa as 16-year-old high schoolers with Sephiroth as the principal? While I understand it's nice to have characters your age and in your situation to identify with, it's really not a good idea to make high-school fics unless the characters are high-school age to begin with.
Good example: (as much as I would hate to see this) Gunbuster
Bad Example: Guilty Gear (99% of the characters are in their 20s or older. Why would they be in the same grade as May and Bridget?)
6) This team needs a female/male character!
So, you like The Outsiders. But, almost every character is a boy! And you really like Dally. I mean, who doesn't? He's soooooo dreeeeeaaaaamy! "Dally needs a woman!" you think to yourself.
So, what do you do?
Let's make a female greaser to join the gang! She'll be the best girlfriend ever and she'll be the best fighter and stuff!
Yeah, this is a bad idea. The characters turn out to be Mary Sues 99.9% of the time. The same goes for adding a male character to a primarily female team, like the Sailor Senshi. They have Tuxedo Mask, and it's canon that only women can be Sailor Senshi. Also, David Gonterman tried and failed.
7) I'm gonna rewrite the series to how I see fit! With more YAOI!
Look, I like yaoi.
But I don't like it when someone decides that "the series was good, it just needed more GAY SEX!"
No, it doesn't. If it's something like Yu-Gi-Oh, the focus is not on how Yami Yugi is going to seduce Seto Kaiba and they have awesome sex! The focus is on card games and friendship (yeah, yeah). Character relationships are important in a series. It's just that a lot of people forget, there is such a thing as friendship! A middle! A grey area! Not every relationship has to be outright hate, romantic, or sexual.
Also, I think it's quite insulting to the original creator to say that "Your work wasn't up to my standards. I'll rewrite it to make it better, but under my terms."
Well, that's all I have for now. Later!