I'm with VirginiaTam on this. The loosely organised ones seem to me to have the best chance of developing into a "special" evening.

No real MC, or at least an MC with a light touch, and a good mix of singers and instrumentalists. Circular seating and taking turns, with enough "standards" sung/played for people to join in with choruses, or pick up the accompaniment. Tolerance and respect for the version that the current singer/ player is doing, and inclusivity for people who might want to try singing or playing but are not as confident as the more experienced participants. Enough people present (and singing/ playing!)for your turn not to come around too often. Good banter as the circle progresses, and a few words from the current performer before or after each song/ tune giving some background or just a humerous story associated with it.

No "themeing"....I think that restricts the flexibility and sponteneity of what's happening. And certainly no "heavy-handed" MC-ing or taking over by a small group.