What behaviors do people do that get under your skin?

A couple major things that get under my skin, that ultimately drove me away from WoW's RP (and WoW in general) are people who suddenly can't RP with you because you're not doing things their way, or they have to go out of character to chide you about what you said in-character -- something they easily could have done in-character. Also, people who take RP romance WAAAAY too seriously. My God, those people... I needn't go into explanation.

What behaviors are common in RP environments?

I'd say the most common behaviors in RP environments are Lone Wolf-ism, Oddball-ism, and Romance-ism (not Romanticism~♪ that's different!). If you see someone, they're either playing McBadass by the bar counter (if you're in a bar), suckin' down those whiskies and oggling the women. Or, they're yukking it up with their little friends, and doing all-around goofy things, such as acting like children trapped in adult bodies. Or, and this is probably the most common, they're talking to their lover.