Hmm let’s start with the negatives here so we can end on a positive note :)

My most obvious pet peeve: lore mistakes, especially when I’m confident that a mistake has been made. They tend to take me out of the story, so I have to focus again before going back in. That being said, just because there are mistakes in a fic doesn’t mean that it’s trash and that I’m going to stop reading. Most of the time, I can still enjoy what the author is trying to convey and see that they love the characters, and that’s more important to me than knowing that Solaria is a city and that it's several times smaller than the Royal Capital/Royal City (both terms have been used in-game to talk about the same place, another fun fact) 🤓.

Second pet peeve would be out of character moments, UNLESS I know this is what I’m signing up for. As in, I used to read fics from an author who would apply certain tropes to the characters that just wasn’t them at the end of the day. But they wrote well and the story was interesting so I kept reading. Most of the time though, if I end up thinking “I don’t think he would say that…” then it’s a bit disappointing. But I get it, characterization is hard; sometimes I reread my own fics and I’m not happy with some of the lines anymore :’D

Now for two(ish) things I do like!

The first thing I love that comes to mind is when the author shows that they really get the character they’re writing about. Doing something that is just so THEM and that not everybody would think about. It’s such a flex lol. I love being surprised in a good way when I read fics.
In the same vein, when a character makes a surprising yet incredibly sexy move and I’m just gagged? LOVE that. Even better when I’m in public and I have to keep my face from contorting.

Also also! I looooooove when authors write stories taking place in the canon universe… but it goes to places canon would never go while still having good characterization! THAT is such a big appeal of fanfiction to me. We’re talking switching, non-canon pairings, unexplored kinks, NPCs getting their flowers, lore extrapolation/deep dives and the juiciest hypothetical ways canon could’ve gone but didn’t. Love that to bits. Now, that’s not to say I see those things as “correcting” canon though. It’s just nice, fun exploration that gets my imagination going :)

I’ll stop there for now but if you want to know about something more specific on that topic, don’t hesitate to ask! Thank you for your question 🥰