I do wonder if someone gives a folk club a try how they might be expected to assimilate all these opinions on manners before putting a foot wrong. I think each club essentially defines its own code of conduct by majority opinion and what is right for one (eg a back room singaround) won't be right for another (eg concert based). But here's a couple of things that I think are important that I tell newbies:

1 If you are listening show common courtesy, don't talk during performance, fiddle with noisy things, switch phone off and only enter the room if you can do so invisibly.
2 If performing - learn the song or how to play the instrument in your own time (use words as a prop only), sing and play loud enough for people to hear and ensure you speak clearly enough that anyone trying can actually hear what the song is about.

Bugbears: 'drummers' who join in with rhythm when not invited; people who say I just learned this today (my heart sinks and is rarely disappointed); guests who don't stay in the room to hear the performances of others

I believe Folk Clubs are fantastic to give a stage to people who are just starting out and generally newbies are given a warm welcome. We all have to start somewhere. What they don't seem to be good at is telling people they're never going to be any good and in continually providing a stage to people who should be kept off it. However, speaking personally it was one of those 'well I can do better than him' moments that got me performing.