Post half naked videos of themselves replying to a video, just to drum up views with sex appeal.
Post butts and boobs on thumbnails of videos that have nothing to do with either.
Start drama. ('did you know so and so is a such and such?')
Whine about pointless stuff. ('omg she didn't like my shirt so I'm going to cry about it')
Grown men and women acting like children. It's not high school, it's youtube.
Screaming and squealing.
Insulting and complaining about what other people have on their channel that has nothing to do with anyone else. ('no one should make minecraft videos, they must be dumb because it's so over done they won't get views blah blah blah')
General rudeness, petty comments, ect.
TL-DR: Immature pointless BS is what I hate the most. I see it in every youtube community I encounter and it seriously needs to stop.