Author has written 6 stories for Naruto, Marvel, Star Wars, High School DxD/ハイスクールD×D, Game of Thrones, and Percy Jackson and the Olympians.
My Personal List of FanFiction qualms:
#1) Loud, obnoxious Naruto: As someone that writes with Naruto Uzumaki/Namikaze in every story, there is nothing I dislike more than a story where the blonde Jinchuriki is a loud-mouthed, naive fool that is always allowing himself to be abused. Seriously, why would anyone be a friend with someone who shoved his fist through his heart willingly? That's not a good character, that's a stupid character.
#2) Static Main Characters: I've touched on this once before, but the idea that a character must remain one way (for no reason) is one of the worst illnesses in the wonderful FanFiction community, particularly when it comes to Naruto. Some very loud people won't accept that the nature of sentient beings is complex. They believe that a good guy must always smile. They believe that a bad guy must always show apathy. If the dark character shows mercy then that's a crime. A hero can't hide anything or else they are fake. There are some characters who stay good or evil, but the extent changes, and that's fine because they grow in some way. Either way, I try to break this trope in my writing, and thankfully I have changed some people's minds.
#3) Mass Genderbends: I think of myself as an open person when it comes to the ideas put in stories. One thing I could never warm up to is the idea that every male around Naruto has to be turned into a female. Why? For the sake of some global harem? Not that I am against any single pairing or harem, because I'm not. Why not put some effort into it and create a female sibling, an Original Character heavily influenced by their counterpart in whatever property they existed in previously?
#4) Fast-tracked Stories: These are the kind of stories where someone will tell another character their life story just because they asked. These are the stories where secrets last a total of four lines in a single paragraph. This kind of writing (in my opinion) takes away from the satisfaction of seeing the payoff of something that's meant to be important, be that a story arc or a romantic interest.