Gamers who like another rpg. Dislike playing D&D yet join the game. Then proceed to constantly point out how much better rpg xyz is compared to D&D.
Missing the point of certain rpgs. I get that some like running rpgs where players start with a rusty dagger. No money to the characters name and wearing nothing but the rags on their backs. Yet decide to run Exalted which is anything but. It possible to kick butt from the beginning with the Exalted as well as having a army.
Gamers who like only playing one rpg. I can respect liking say D&D. I just can't play or run D&D all the time I need a change every so often. If as a gamer one hates Shadowrun why join the game. Seems like a waste of time.
Gamers who like playing characters with high stats and really low dump stats. The 20 Con and Str with 5 Int and Cha types. Who then want little or no penalty. Or who try to get around it by pumping points into skills. I used to really dislike that. Even disallowed that in my games. Now I made a new houserule. Put a 5 int and 5 Cha. The Str and Con are frozen at 16. So go ahead and pump as many points into social skills at first level. Your character is offscreen roleplaying going around talking to people and getting better in social situations at the expense of becoming tougher and stronger.