These are things I learned in fandom. In fact, I'm guilty of that first one myself (b/c I was a n00b lol). In fandom, the worst that can happen is that fellow fans get annoyed and don't read your fanfiction. But in publishing, people not reading your books can cost you considerably more.
• Don't use excessive epithets.
In fandom, epithets are both profuse and diverse. This is not a good thing. The most common was writers using either a character's hair color or nationality as a stand-in for their name--you know, to "vary" things up a bit since saying that person's name over and over again is apparently no good.
As a result, writers "creatively" used epithets such as "the pinkette" and "the silverette" to describe characters with pink or silver hair, which happens often in anime fandoms. Weirder yet was when a person with black hair got turned into a bird. Rather than saying "the raven-haired bloke" (which I'd still advise against), it was just "the raven." You have no idea how strange (and hilarious) it was to read lines like "The raven nodded and walked away" or "He kissed the raven."
Likewise, nationalities: "Toma liked tea well enough, but it wasn't the Japanese man's favorite drink."
This is fine if you're telling the reader for the first time that Toma is Japanese. Not so fine if "the Japanese man" becomes his second name continuously throughout the entire story. Or if Toma is the narrator.
• Keep characters IN character.
Yes, it's possible to write your own character out of character.
There was this disturbing trend in m/m fandom to turn one half of the pairing into a pseudo-female. In canon, the guy could be crass, loud and short-tempered, but if he had large blue eyes, he was doomed to be a blushing, stuttering, passive bottom to his manly top. Cue groan.
What a lot of writers tend to do is come up with this exciting scenario in their heads and then proceed to write it without any regard to how the actual characters would react given that situation. The author is so enamored of her "great idea" that she fails to realize the characters would never actually behave the way she wants them to.
This can happen just as much in original fiction as fanfiction. We create our own characters, but then we have to stay true to how we portray them. Readers will pick up on inconsistencies. And if the inconsistency is deliberate, then there has to be a reason why.
Part 2 next Monday. Have a great week, all!