Who am I? I'm no one. In the world of writing, I'm nobody. I'm not professional, I have no authority to speak of. So you need not concern yourself with my opinion on your writing.
That said, if you want your fan fiction to get positive feedback, you'll have to keep your audience in mind. These are just a few standards I (a zero) hold fan fiction to. Some things you may want to avoid or capitalize on, when writing.
Rule 1: Canon - The Golden Rule:
If someone or something is wildly out of place and this is not done correctly, it's non-canon and at worst is a fan fiction of a certain series in name only. If it's a humor fic or something like that where it's not to be taken seriously, then I can let it slide. This goes a long way toward explaining my feelings on yaoi and yuri. Alternate universes are all right, just don't go crazy with it, you know?
Rule 2: Yaoi/Yuri:
I don't like it, though I don't have any problems with homosexuality. My beef with yaoi/yuri is that it's not canon to the characters (ignore this if, again, you're doing a comedy story). I'm just saying, if I were a creator, I wouldn't appreciate my two straight characters suddenly becoming schoolboy lovers. You can be best friends without being in love. It's the canon that matters. I mean if they were gay, lesbian, bi, or whatever in the actual work then fics that portray them as such are fine by me. There needs to be supporting evidence for the pairing. Good evidence.
Rule 3: Summaries:
I don't ask for a lot in summaries, only a little bit. But never ever say "I suck at summaries". You might as well be hanging a neon sign that says "THIS FIC SUCKS" or something like that. Why should I like your story if you don't even have confidence in your own summary? It's not that hard if you just try. Also, spelling. If you have bad spelling or grammar in the summary, it's a bad sign that it won't be much better in the actual story.
Rule 4: Spelling and Grammar:
If English isn't your first language then I can forgive this, but otherwise it's not hard to look things up real quick. I can forgive genuine accidents, I mean we're all human. But if you're consistently confusing "bye", "buy", and "by" then there's a problem. Look:
Bye - "Bye, see you when you return."
Buy - "I would like to buy a Shamwow."
By - "He stood by the river."
There - "There are no dragons in here."
They're - "They're about to start the movie."
Their - "I like their house."
Where - "Where is my cake?"
Wear - "I wear pants."
It's - "It's too late for breakfast."
Its - "The dog ate its dinner."
To - "I am going to start a fire."
Two - "There are two puppies."
Too - "This porridge is too hot."
Then - "I will eat, then I will brush my teeth."
Than - "He would rather play than get back to work."
Your - "This is your book."
You're - "You're about to go on a road trip."
Less/Fewer - "There are fewer bananas." - "There is less room for the bananas." (See how it works with placement of plurals and is/are?)
Besides, if you don't take the time to spell or use proper grammar, then you just look stupid.
Rule 5: Criticism:
No, flaming someone for their story is never a good idea. If it's just that bad, you still don't have to be terribly rude. But that's not my problem. The problem is a lack of criticism. If there is a problem in an otherwise good story, say so in a review. And if you're the author, encourage constructive criticism. I for one like to know my flaws so that I may work to improve them. If a writer can't take a piece of genuine constructive criticism and insist on only flowery comments that sing their praises, then they are being an immature diva who needs to grow up. Criticism is positive AND negative.
Rule 6: Shipping:
Good night, what is wrong with people? They're fictional characters. You think if you pair someone up romantically and hate on another character in your story, that will convince the creators of the real work to conform to your romance? What? All of you chill.
Rule 7: Don't Like, Don't Read:
Bull. This is a telltale sign of those immature divas from Rule 5. If you put this before the criticized material, I can't say too much against it. But if you say so after someone says "this sucks", how on earth do you think that's a good retort? "Oh I'm sorry, let me just go back in time and not read it!" And besides, if you really hate getting criticized, take your own advice and don't read the reviews that you don't like.
Rule 8: Song Fics:
No originality, you're just posting the lyrics and sprinkling a few bits of your own stuff in. If you remove the lyrics, the author has hardly done anything. If I want lyrics, I'll hunt them down. Or just listen to the song on another tab while reading. I have been a bit guilty of this on one occasion though, but I realize I could've handled the story differently and if I rewrote that story, I'd handle that part differently.
Rule 9: Cursing:
If you must, I won't stop you. I myself don't do it without censoring (personal reasons, I've never believed in cursing). But just remember that dropping the more colorful four-letter words gratuitously will not work. Especially if it's out of character (which goes back to canon). Unless it's played for laughs or explained in-story, I cannot imagine a fic where Alphonse Elric starts dropping the F bomb whenever something goes wrong, I wouldn't take that fan fiction seriously and dismiss it as bad.
Rule 10: A Girl is Sucked Into...:
NO, NO, NO, A THOUSAND TIMES NO. I don't care how well you self-parody or subvert the norms, I will never read the stories where a girl finds herself in the universe of a work. It's thinly disguised self-insertion and is a stone's throw from Mary Sue. The same goes for fics where a character suddenly has an unexplained sister or other female relative show up. You can try to make this work, but it's an uphill battle and I wouldn't recommend it.
Rule 11: Mary Sues:
I think we all know this one, I shouldn't explain.
Rule 12: The Under-Appreciated:
There are a million fics about Character X, but hardly any that are about minor recurring character, Character Y. Before you write about Character X, think about Character Y. This stems from my appreciation for the characters who tend to keep popping up but never get much spotlight.
Rule 13: OCs are OK:
They're not automatically Mary Sues, they just have the most potential to be one. It's a tricky subject, but it can be pulled off. Best to not make them related to canon characters though. (I did used to have my character related to a canon character, but I retconned that once I realized it was a bad idea.)
Rule 14: Idea and Execution:
It's my belief that any idea for a story can work and be done well. The thing is just that some are much harder to pull off and shouldn't be attempted by everyone. I mean execution is really all that matters. Two stories can have the same plot and characters, but the execution will decide which one I'd like better.
Rule 15: False Modesty:
Don't go around saying "this sucks, don't read it" in your own work about 15 times or so. If you really don't want people to read it, then you would have never posted it in the first place. Self deprecation isn't wrong, but it gets to a point where it seems like false modesty just so people will shower you with compliments and assurances that it's a good story when you already knew that. And if you're genuine in your heaping amounts of self criticism, then... dang. You need self esteem class or something. Long story short - If you really, really loathe your work, don't post it and tell people not to read it. It's up for a reason.
Rule 16: The Mike Weiss Contradiction:
"It's better to have an ok project done than an amazing project never finished." NO. It is not and it will never be okay. I mean, if you're really tired of working on something and it's really just minor, then sure you can kind of slack, it's only fair. But still you should always strive to give your best work. Your name is associated with this, and what good is it to get something quickly if it sucks? To do this, it deprives the audience of quality work and makes it look like you assume they're dumb enough to sit through anything. An audience can only take so much. On top of that it deprives you of a work you can be proud of as well as a challenging experience.
Rule 17: No Masters:
If you think you're great and don't need to improve, then you're doing it wrong. Writers can always improve. Take on new challenges and diversify your experience. Get out of your comfort zone. There's really no risk. If it works- it works. If it fails- at least you tried, and now you have a better grasp of your limits.
Rule 18: Rereading:
After writing a story/chapter/whatever, it's best to read it real quick. You can catch errors and re-evaluate whether or not you're completely happy with it. If you can have someone else read it for you in addition, so much the better. But if not, just do it yourself real quick. Also, after rereading, try reading the final sentence, then the one before it, and so on until you get to the first. It's easier to catch mistakes that way. Someone told me that, and it really is a logical concept.
Rule 19: Genre Probability:
Let's talk about Invader Zim. The universe of that show is very bleak and cynical. Almost everyone is an idiot, a jerk, or both. And you can count on most of those people to have terrible hygiene. Everyone more or less hates each other, and if anyone died, no one would be missed. Despite all that, it's still played for laughs and is a dark comedy. That's how Invader Zim rolls. With all that in mind, writing a heartwarming fic depicting the IZ universe as anything less than a hateful rat den comes off as kind of out of character for anyone involved in the story. What I'm saying is that keep the genre of the original work in mind whenever you feel like writing. It's not impossible to pull off a fan fiction of wildly different genre, but there's a gamble on how credible it will seem.
Rule 20: Editing Before Editing
Someone told me this, and really it makes perfect sense. But I am putting it in my own words.
Having a beta reader or an editor doesn't mean you can give half effort and expect them to clean up after you. They have other things to do, so they can only give so much focus to your story. So make their time worthwhile by fixing your grammar, spelling, and punctuation errors before sending it off. That leaves them with only the more internal problems, like plot direction or characterization. The 'smart' mistakes. Eliminate the 'stupid' mistakes on your own, so that your editor can have more time invested in helping you fix the 'smart' mistakes.
But even if they don't have other things going on, fix your 'stupid' mistakes anyway. Let's say you're a beta reader with nothing else going on in your life at the time. Would you like to trudge through a story with poor grammar that you're expected to point out, and hopefully remember to notice deeper problems? Or would you rather read a story with wonderful spelling and grammar, leaving you free to discuss important problems?
Rule 21: Know the Fandom
I didn't really cover it, because I thought it went without saying. But here it is, just in case.