By order of appearance:
1. Lots of spelling and grammatical mistakes. Sometimes it's even worse than MTL.
2. Writing style doesn't agree with me: lots of dialogue, script-like writing, not nearly enough scenes setting. Picture two heads talking to each other all the time.
3. Plot going nowhere: writing each chapter as they come with no clear set goal. It's fine in the beginning, but it eventually turns into a chore to read for me, because I can't stand slice of life and need action and adventure.
4. Finally, if everything else is great, the MC can ruin the whole story. This is really subjective, however.