1. Formatting, spelling, and grammar all matter. An occasional mistake or typo is okay, but nobody's going to read your story if it's illegible.

2. Pay attention to continuity. If character A tells character B a secret, then a paragraph character A tells character B the secret again, you lose the readers.

3. Use a spelling checker. Have someone else proofread your story; you can't see your own mistakes. Someone else who knows more than you do. Use punctuation correctly. Use the right word, not some other word that sounds kind of like it.

4. If you're going to be specific about something -- medicine, technology, weapons, building houses, fixing cars, anything at all -- do your research. Make sure you've got the details right. Make sure you've got the terminology right. Because while you might not know the difference, some of your readers will know, and that will break the story for them. They'll probably point it out to all the rest, too. Remember, research is your friend. Use google. Use wikipedia if you have to. When in doubt, research it. Professional writers have to research, even fiction writers. You can't just wing it.

5. Know the canon you're writing about. If you've seen 3 episodes of some TV show, don't try writing in that fandom just yet. If you've seen a movie based on a book, or many books (or manga, games, whatever), read the books. Don't take other fanfic as authoritative; the authors may just be as clueless to the source material than you are.

6. Keep the period of your story in mind. Slang changes; research period slang if you're going to use it. Pay attention to how people address each other.

7. Once you set up a premise, LIVE WITH IT, don't just ignore it when it becomes inconvienient to your plot. Really, is it so hard to have a bit of internal consistancy in your fic? Did you forget your own backstory?

8. Show don't tell. If you have to have one character talk about how smart, tragic, sweet another character is, chances are your character hasn't demonstrated any of these qualities.

9. If you are going to have your characters do something that doesn't come with its own canon justification, then you have to justify it yourself. Get into the people's mind and come up with some logical reason for them doing what they are doing. Manipulate the backstory. Juggle the circumstances. Write some plot. Create a character arc. It's really not that hard, folks. Especially with pairings, all you have to do is convincingly point out how the characters are attractive to each other and give them some set of circumstances where they can act on their attraction.

10. In text author notes are stupid and annoying. They interupt the flow of the story and are very jarring. Author notes should be kept short, if included at all. A thank you to those who read the story and a gentle request for fanfic are okay. We don't need to know that your dog ran away, you're failing math, your parents hate you and you cut yourself. Save it for Livejournal.

11. Avoid fandom slopover. Don't assume that a given fantasy creature is going to be the same across the board. For ex, Tolkien's dragons are very different creatures from the ones in the Pern novels. For starters, Tolkien's dragons are always evil and certainly can't be controlled by humans. McCaffrey's dragons are far more benign and routinely serve humans as mounts.

12. Please read your story once over before posting. If it doesn't make sense (literally, not metaphorically), don't post it.

I might add more.