Inconsistent grammar and punctuation is a big one for me. Especially with run-ons that have limited punctuation.
Showing not telling is also important yet one I am still working on. It is a rough mountain to climb and requires some serious commitment.
Also, big changes in stories that are very sudden. While I think plot twists and major events are expected, when they are consistent with absolutely no rhyme or reason -- they are annoying. Also, I heavily dislike characters that follow a single trope to a tee. I don't mind if characters follow certain tropes or patterns. But my goodness every chatacter feels (and is drawn) the same for certain fantasy (in particular) webcomic or story. They are just rotated around webtoon by webtoon. There is no originality.
Also, what's with characters in writing only being the ages of 17 - 22? Maybe that's a more me issue as that's what I see often, but I would love to see main characters in their mid 20s, 30s, etc.