1. Silly Pen Names. Your pen name is your initial vehicle for communicating information about yourself. You want your readers to think that you are sophisticated (or at least not ridiculous). This means avoiding anything cutesy, plagiaristic, or overenthusiastic. Do not use the name of a famous person or a character in your chosen fandom to refer to yourself. People who name themselves after their favorite characters are otaku (in the uncomplimentary sense).
2. Gushing. Gushing is the best way to let your readers know that you are cutesy and overenthusiastic. Don’t get too excited about things, especially things that aren’t actually part of your story (such as the disclaimer or the summary). Eliminate extra commentary—don’t try to make clever jokes with your disclaimer, for example. Any time you put an exclamation point somewhere, double-check to make sure that it’s not making you look like a pre-teen.
3. Apologizing. Do not apologize. Rather, if you have done something wrong, correct it. (In the event that you have done your very best, but you feel that your work is still flawed in some way, an apology may be acceptable. However, this is necessary only in rare cases.)
4. Incorrect Spelling of Characters’ Names. Consistently misspelling proper nouns in fan fiction is just sloppy and disrespectful. I don’t like to read stories whose authors have so little reverence for the fandoms in which they choose to write.
5. Poor Editing. Apparent problems in grammar and punctuation can be a matter of chosen style; in any case, honest mistakes are not a “quick fix”. However, misspellings, typos, and obvious grammatical errors are signs of shoddy work.
6. Plagiarism. You would naturally never quote something word-for-word without crediting the author, but paraphrasing without acknowledging your source is plagiarism, too. Any plotlines, characters, or turns of phrase you find in other places should be credited, unless they are in the public domain. If you don’t know whether something is in the public domain, credit it. It’s better to be overzealous in this matter than to be thought rude or—worse—a liar and a thief.
7. Unfinished Stories. Please don’t publish your story if it’s unfinished. There’s nothing more cruelly disappointing than loving a story and finding that you’ll never discover how it ends. Wait until you’ve finished it. If you must publish it in pieces, please post a note that saying when you intend to finish. If you don’t intend to finish something, please tell us that, too.
8. Misuse of Foreign Words and Terms. It’s sloppy, and it makes you seem overenthusiastic. I despise this particular atrocity so much that I’ve devoted an entire essay to it here.
9. Mondegreens. According to dictionary.com, a mondegreen is “a word or phrase resulting from a misinterpretation of a word or phrase that has been heard”. Mondegreens are usually formed from song lyrics (perhaps because it’s easier to misunderstand a singer than a speaker), but they exist in fan fiction, too—for example, writing “a head of the pack” instead of “ahead of the pack”. (That one could also be a typo, but I’m having a hard time of thinking of a good example.) I suspect that these happen when authors write phrases they’ve heard but never seen in print. You won’t notice these on your own (for reasons that ought to be obvious), so if you’re not perfectly confident in your command of the language you’ve used, please have a friend proofread your story.
10. Overuse of Specific Words. If you use a word too often (especially a particularly noticeable word), readers will be distracted. You can use the “find” tool on your word processor to avoid this; simply look for any words that you find interesting or unusual (also known as "vocabulary" words), and search for other times you may have used them in your story. An easier way to do this is to use the tool located here.