The essence of a good singaround seems very complex - almost a microcosm of life!. Thankfully everyone (in a singaround) is different and can bring that variety to the event and I think that the different characters, approaches to performing, musical abilities, favoured types of songs or tunes help to make a session engaging for everyone. And engagment is key to a good session IMHO. If you're engaged in the activity then you probably enjoy it and want to sing/play along (when appropriate) and want to come back. In the past I have found that a core of (understanding) good quality performers helps to set a reasonable standard but doesn't stop new people performing.
Regarding MCing or not and if so how to MC I think there needs to be some gentle guidance but low-key and with the respect and consideration you would like to see shown to everyone there.
Isn't, though, the secret really about finding the session that fits with your expectations of fairness to all and enjoyment of singing and playing? Your folk niche?
I suppose too I'm a believer in giving feedback both positive and negative. Positive is sometimes not given because it doesn't seem necessary, negative is sometimes given in the wrong way or is taken badly. But maybe this is also the secret of creating and maintaining a good singaround - as MC to encourage comments from the people there perhaps during the break and as a participant to say what you think to the MC? This may be a rather ideal scenario and I would be the first to admit that in practice, of course, it can be very difficult to handle certain situations. Good thread!