I am a painfully annoying critic, but a fairly consistent one.
-My main points are usually about legibility issues and usually come along with the recommendation to put a spacer between every 3-4 sentences to break up blocks of text into something more legible and phone friendly.
My second biggest point is when people provide unsubstantiated and incorrect facts about things most easily checked with a search online. I don't mind if people get something wrong, but I'm not cool with people claiming things that the first search result in a search engine debunks immediately.
My third biggest issue is with people adding current real life political controversies into their stories. People read partially to escape the real world or get a break from it for the duration of the story.
It is extremely annoying when someone spams a current political controversy in a story having chosen one side or the other. I'm not here to read anyone's manifesto about their conviction on which side of a controversy is right. I'm here to read whatever story they are writing a fanfic about, not to hear their characters rant on for pages on end about rl stuff instead of progression in the fanfic itself.
I find it extremely offensive, insensitive and antisocial when someone shoves their opinion on currently hot topic rl controversies down another's throat in the shape of a characters rant in a fanfic. A snarky comment or remark? Sure. But not a chapter long or more rant.
Last but not least is pre-teens following 18 scenarios, and explicitly detailed ones at that. It rarely happens on this site, but such content should not be encouraged. In addition, I think regular NSFW content should atleast be propperly tagged in the story with a beginning and an end, so people can skip such scenes as a common courtesy.
Something I would like to see more in stories, is for once an MC that is not abused or living a horrible life beforehand, or if they are, to atleast not spend entire chapters detailing abuse.
I find it highly disturbing how often something like a HP fic spends a great many chapters on detailing child abuse at even larger escalated levels than Canon HP, like its some sort of sick fantasy of the author that they are writing out.
The same goes for actual torture scenes. A summary or shot description, sure, but unless your story is specifically about abuse or torture, then I recommend focusing on the actual point of the story, or the experience you wanted to deliver.