Hey, I'm relatively new to the game and I don't have much RP experience in video games (and sites like tumblr), only some basic Pen-&-Paper experience, but I'll try:
What behaviors do people do that get under your skin?
Making extrem mary sue-ish characters is pretty annoying, though I have to say in their defence that making a slightly (note: slightly!) mary sue-ish character can sometimes happen faster than you think, especially when the character develops. But that's pretty dependant on the universe/situation so I can't say if that's true here, too.


What IN CHARACTER behaviors are common in RP environments/situations?
One thing I often noticed during Pen-&-Papers, is that people (myself included) struggle with playing out negative situations of their characters. With that I mean especially emotions like sadness (in extreme situations, like character death) and fear. I think situations like those are tricky to play and I myself find it pretty difficult to play a scared character convincingly.