Olaf the Stout
Hero
Players cheating is right up the top for me. I also don't look too kindly upon PC's attacking, stealing from or otherwise destroying party unity with the excuse of "I'm just playing my character".
DMs playing favourites or punitively punishing players is also up near the top (especially punishing them in-game for out-of-game issues) or completely railroading a game, to the point that any other solutions to the problem besides the "one true solution" basically auto-fail.
As someone that hosts our game and has so for over a decade now, disrespecting my house it a big RPG crime for me. Don't throw your bag down against my wall. If you have any rubbish, put it in the bin, don't leave it on the floor next to you. If you drop a deuce in the toilet, clean up after yourself. It's just basic decency.
DMs playing favourites or punitively punishing players is also up near the top (especially punishing them in-game for out-of-game issues) or completely railroading a game, to the point that any other solutions to the problem besides the "one true solution" basically auto-fail.
As someone that hosts our game and has so for over a decade now, disrespecting my house it a big RPG crime for me. Don't throw your bag down against my wall. If you have any rubbish, put it in the bin, don't leave it on the floor next to you. If you drop a deuce in the toilet, clean up after yourself. It's just basic decency.