Whenever a group of people get together to do ANYTHING, there will be disagreements.
Over the years I've noticed most of the incidents that have been mentioned here, and I too get bugged when someone shows up just long enough to do their one or two songs and leave.
Another thing that may be causing people to leave is the few people who sing the same 2 or 3 songs at EVERY session. How do they avoid getting bored? How does the audience avoid getting bored?