I mentioned the same in another thread like this one, but people who expect everyone to do bossing annoy me. I get that it's a big part of the game and that a lot of people do it, but there's no need to assume everyone likes/does it. I personally hate PvM and nothing annoys me more than someone just asking me what kind of bosses I do out of nowhere or telling me to go bossing.
Another thing that annoys me, and it applies to both real life and RS is people who complain about something just because everyone else complains, without even trying the said thing out. This is very common thing in RS especially, where people cry about every single possible update just because everyone else does without even really having an idea what it's about.
Last but not least, overly talkative/social people. I'm a kind of person who likes personal space, peace and silence around me and I find it extremely frustrating when people keep talking to me a lot, especially if it's about things I don't care about.