(I've had a bad day, so to release steam, I need to bitch about some things. I'm new to this as well.)
I hate roleplayers who think they're the most professional at things, that say you have to be super elite or something to join their roleplay.
That doesn't really happen on this site much, but believe me. I've seen it.
Also, I dislike to deal with impatient roleplayers. For example, they don't even read the previous posts on the roleplay, and write something completely fucking different than what the others said, or people that just expect you to post a OOC when you have the interest check. Like they'd read your comment about you making it, but they'll be like "Any day now.." It's rude, and I understand that you like the roleplay and want to make the Character Sheet and post as soon as possible, it's rude as hell.
Speaking of rude and impatient, people that post twice when the roleplay just started. Some people don't really know that it's polite to wait when it comes to that, which I get. But for the people that know to wait until everyone's posted their beginning posts, it irritates me so much.