I agree with much that has been said earlier. Certainly a light touch by the MC but also fairness, so that everyone who wants to gets a turn.

I would add a core of regulars who are good singers with interesting and varied repertoires.

I sympathise with Shimrod's point, but the occasional poor singer is a small price to pay for a session based on the principle that everyone who wants to have a go should be given the opportunity. For every poor singer there are usually many competent singers, not forgetting several good ones. To me it is more important that the newcomers should be encouraged and given support to help them overcome their anxiety and give it a go. I think a singaround is essentially a social event where people are sharing songs, where the distinction between performer and audience is essentially non existent, so everyone is both performer and audience. Yes there are a few who simply come to listen and they are generally made welcome, but they are a small minority.