Newer bands often do things that are "cool" but deadly. One is to pick inscrutable tunes -- so inscrutable that noone is quite sure where the A part is. I would probably encourage them to listen to some of the seminal Canterbury orchestra/Yankee Ingenuity type albums and start with the chestnuts -- there's a reason those tunes became "canon." Listen to how the tunes are matched -- what are the keys, the personalities? Why do those medleys work? There are newer dance-length CD's by longtime bands that are also excellent. If they already have their own favorite repertoire, have them at least screen out the old time tunes where the A and B parts are different by only one note -- and the Celtic tunes where they're different by all 64. A good dance tune should have a recognizable shape, be kindof singable -- have a good mix of note values, neither all sparse quarters or relentless machine gun sixteenths.

I also think people who insist on doing that when it freaks out their neighbors are valuing their own fun more highly than the comfort of other people there and are behaving in an anti-communitarian way - which is their perfect right, but it's not an unalloyed good. And some of the people who are freaked out are freaked out because if somebody they're not expecting comes at them they think somebody (maybe them) are in the wrong place and their anxiety level goes up. Not homophobia - just hanging onto the dance by their fingernails. That's a good thing to be aware of when you're swapping sides in gendered contra land.