I can't stand when apart from arena or specific RPs where that's a gimmick a GM let's a PC kill another PC when neither have broken any rules. Also dislike when GMs give their characters more abilities unique races, etc. I was looking at what seemed to be a large Free RP, but the GM seemed terrible. They had Main Characters who were more less just the back bone of the story not the PCs, and had rules like no weird powers. Also a required anime picture of your character. I really hate GMs who make you provide pictures/drawings of your character. If you're not an artist it limits you to what you can find online. Just letting off some steam.