Oh, and this may not be the worst kind of rudeness, but I do consider it kind of rude to get up and leave a song circle the minute you've had your last turn 'at bat'.

Sure, sometimes you have a legitimate reason for leaving, and if you're the last one to get a turn in a large group, I can see holding out till you've had a chance to do your song.

But a lot of people do this routinely, at open mics, song circles, singarounds, jams, etc.   And this rudeness is at its worst when X, who's the informal leader du jour, has already had a couple of turns, while latecomers have either had none or maybe one, and X does his 3rd song and not only leaves but says, "Hey, it's late. Why don't we call it a night?" right before it's one of the latecomers' turn.

That sort of thing happens way too often, in my experience.