People who get invested too much on other people's tastes. It's cool to talk about what people like and don't like, but when people get all up in arms if they play a game in a style they don't, play on a platform they don't, or just do anything they don't like...that gets weird. The older you are the dumber you look for doing that in my opinion.
I used to be guilty of this in the past until one day I realized how silly I looked. Why should I care of someone doesn't enjoy something I like and vice versa? Again, trading points of views and actually discussing gaming can be fun. However, I find that 99% of the time "discussions" devolve into screaming matches or outright negative sass like a political discussion on television.