One thing I find mildly annoying (as a non-performer) in the open-mic nights at the acoustic club I go to is that there are often several people chatting away at the back of the room while someone is trying to sing at the front - and these people are usually other performers waiting their turn. I do recognise, on the other hand, that it is a sociable event for the loyal regulars to meet and chinwag at and not necessarily a place for complete silence and rapt attention, but it does seem a bit disrespectful in a concert-style room. (And it costs £1 to get in!)
Background hubbub doesn't bother me at a much noisier session I have been to a couple of times in Bradford... the room is a normal working pub bar with normal people yakking away, and the session of around ten people sits around their regular table in the corner and sings to each other. Here the singing seems integrated into normal pub life, and you don't expect to hear the singer from more than a few feet away.