Gaming (PC elitists and fanbases of specific games) and MTG.
When it comes to gaming, I hate PC elitists, and the hardcore fringe that every single game seems to have. I hate the mentality of, "if you aren't doing it this way, you aren't doing it right." When it comes to gaming, some people don't give a shit about things like mods, customization, or higher performance that the PC can get over consoles. I for one prefer a console because it takes less effort to set up and I don't have to really plan ahead on building it and doing research. If I want a console, I just have to see what version and how much memory I would like, that's it. Some people may enjoy the plethora of options available in PC gaming, I for one do not. I am too lazy to really look into it. Also, I do not care that PC's may perform better and have better graphics. I've never really had an issue with how games looked or performed on my Xbox.
When it comes to specific games, some games, like Destiny, have incredibly obnoxious dedicated fanbases. With these people, there is only ONE right way to beat this raid. If you do not know exactly how much DPS each weapon does on PvE vs PvP, then you are a casual. The community is one of the main reasons I stopped playing Destiny. It requires too much cooperation with people who I can't stand.
As for MTG, it just seems like the more experienced you become, the more of a prick you become as well. A lot of the better players at tournaments I've been to over the years (be it FNM, a release, or whatever) kind of seem rude. I also hate that it always seems like when someone gets serious enough, they have a need to adopt this stupid little mannerisms that a lot of MTG players do. One thing that annoys me to no end is when people quickly flick or shuffle the cards in their hand.
Overall, most experienced people I met while playing MTG just seemed like pricks. I remember there was one guy who was one of the best players in my state, and everyone said he was a cool guy. I played him, and he seemed just as rude as the other players. If I messed up on something, he would get kind of aggressive, and he made it seem like he wasn't really paying attention to the game, instead talking to his friends down the table. MTG - another hobby I quit because the community was full of pricks.