Interesting point raised above about audiences joining in with the whole song instead of just the choruses. If I'm singing a well-known folk song then I don't mind, as the singer. Generally, I'm happy for people to join me though, as it means they're happy too - then everybody's happy, which has to be good :-)

I don't normally find it a problem if I'm singing a more 'serious' or reflective song - people generally seem to know when it's appropriate to join in or not.

But if I'm in the audience with a guest artist, I find it annoying if someone else in the audience near me (usually a fan of the artists who knows the words inside out but can't sing for toffee) sings along with the verses and drowns out the person I've paid to see - and listen to.