1. There is more than two emotions: happy and sad.
To your readers, the characters are real people. They have emotions, pet peeves and more than one trait that defines them. Are you just a go-happy smiley person all the time?? Are you sad all the time?? No, your emotions fluctuate depending on the circumstances; characters are very much the same way. They won't always be happy and they won't always be angry. Try to show a variety of emotions like anxious, furious, bored, listless, indifferent, etc.
2. Keep it in character (with exceptions).
There is a reason it's called Fanfiction. The basic personalities of the characters have already been given to you. Use it. Don't make a character evil or vicious when he's happy-go-lucky. Don't make a character depressed and listless when he's optimistic and hardworking. There are exceptions to this like alternate universes or in hetalia's case, 2P's. But again, some 2Ps have been given some characteristics and try to stick with those.
3. SHOW NOT TELL
This is a problem I tend to see in most fanfictions, sometimes including my own. The problem is you are writing from the scene in your mind, but you're not telling your readers what you are seeing. You may see it clearly, but your readers can't see inside your brain.
Check out the difference from
"Italy was panicking. He fumbled with the key as he tried to unlock the door. He got it open after 4 tries."
"Italy's breathes grew shorter and shorter, his mind buzzing in panic. His hands were slick with sweat as his butterfingers tried to keep their hold on the slippery snake of a key. Unsteady and shaking, he gripped the doorknob tightly as he attempted to shove the piece of metal into it's slot. Once. Twice. Again. And again. Whining in frustration, he tried to force the unruly key into it's proper place. Click. Finally, the door complied to his efforts and swung open."
I know the differences are extreme but you see what I mean.
4. Write what you like.
I know if I force myself to write a different genre, it will be far inferior to what I write when I feel in my element. If you like writing reader inserts, then that's great keep writing. If you like writing self-inserts, awesome, it's your story. If you like writing yaoi, yuri, hetero, love is love and I don't judge. Don't be discouraged to write what you love because other people don't like it, or it's uncool.
5. Write from the heart.
It's simple. Don't make it cheesy. Write about real emotions, rather than a prince coming to save the peasant or princess from the oh-so-scary villain. And if you are going to write about fairy tales, try to show the underlying thought processes of the main characters. Write what you feel from the heart; down to the anxiety that you feel when in love.
7. Critiques are friends, not food.
Remember you should be writing in proper English, or Spanish, or French.. or whichever language you write in. Don't be afraid of criticism, they usually are meant to help you improve. Listen to the readers when they say "Please, check your grammar." or "There are so many mistakes." There is no shortage of editors and I will be happy to beta for anyone who needs a betareader.
8. Stick by your writing.
Be proud of what you wrote. You have often heard the saying "the harshest critics are often the writers themselves," and it's true. It's okay to be humble, but don't berate yourself too much.
Summarizing the word of the wise, Every story is good, and Every story is bad.
There is no such thing as a perfect story because there is no such thing as a perfect life. Literature reflects life... or does life reflect literature?? I don't really know.
In conclusion, don't be afraid to edit or erase a story and start from scratch.
Sincerly, a fellow fanfiction writer
PS: Find your style of writing, and all the above will come to you as easily as kittens to a bowl of milk.