1. Passive players. They sit there doing jack nothing (not even shooting) and take credit for the victory. In all honestly (and forgive my use of a soapbox), it was usually the player like me, who charges while screaming and firing wildly, who takes the objective/captures the hill/annihilates the other team.
2. Lazy players. People who put no effort or cash into airsoft. If you show up in civilian clothing, at least play well. Playing airsoft with no intention to get proper equipment or improve as a player is like showing up to a martial arts program and telling the instructor that you just want to punch people.
3. People who make sport-like rules. It harms the unique feel of airsoft. If I wanted a sport, I'd go back to paintball. It makes it hard for me to improvise solutions and be myself (I have one of the most unconventional playstyles you'll ever see... lots of charging, over-the-top violence of action, Puzikas-style reloading, taped mags, improvised shields, and flank routes) when anything that isn't specifically covered in the rules is considered illegal.
4. People who don't get the logistics of having even a simple game. They always ask for night games, but have no clue how much planning, additional equipment, additional danger, and butthurt is needed for a successful game in pitch-black darkness. Do they not realize that daytime games are hard enough to organize?