I end up reading A Lot of web novels for work, and my current Absolute Least Favorite things to see are
Relatable Pathetic Lead - The lead whose main characteristics are "eats a lot", "is stupid", and "somehow everybody loves them or they need to save the universe". The dude version of this is usually horny, the girl version is usually clumsy and easily flustered. Especially in a drama, it really throws me off if the main character never seems to take things seriously or work towards things and that's not treated as a conflict. None of those are bad traits. Like, Luffy from One Piece may be a hungry dumbass, but he's also a pretty complicated character, if a very simple person.
Honestly, I'd rather have a character with no flaws than somebody who seems genuinely irritating and incompetent with no particular interesting points beyond a generic good heart and it's always played as cute and funny. It's supposed to be a 'relatable' thing, but isn't that... pretty harsh on your readers. I want leads who are active and driven, or at least inactive for interesting reasons!
I prefer my incompetent characters to be stone cold, unrelatable weirdos with baffling side skills. Like the entire cast of Noazaki-kun.
Surrounded by Props - My biggest nemesis is Dude Isekai where everyone just stands around to make the main character look good. Every lady is helpless and in love with him, or an evil seductress (and also in love with him). Every dude is a irredeemable asshole or the main character's lackey.
It speaks to a worldview I find genuinely creepy and upsetting, like the author can't imagine seeing people as anything other than "mine or in my way". (To be clear, this isn't limited to dude isekai, but it is where I see is the most) I also worry about young readers who read nothing but this stuff and talk about a character acknowledging flaws or apologizing as weak and uncool, and people who don't suck up to the lead character as evil.
The Trauma Chain - I like stories dealing with how we react to extreme circumstances, but so many authors seem to attempt to accomplish this by bring in a new trauma every five chapters. People can spend their entire life recovering from one terrible experience, but protagonists in these stories can lose their entire family in a fire and then, at the end of the season/book/arc, the author declares it dealt with and moves on. It's especially frustrating when a character I really love dies, and it's supposed to have this huge impact, but then they're just never mentioned again and nobody seems impacted by it. American TV seasons are particularly bad about this IMO.
For a good inverse, I love in FMA that Nina remains a guiding force for Ed and plays a role in his final decision at the gate. She's a side character, but, even if they didn't say her name, her impact stayed.