I think someone "in charge" - especially at festival singarounds where the people in the room tend to come and go is a must. There's nothing so soul destroying as being first to get to the event only to find that as the room fills up then your turn gets further and further away because you're sat in the wrong place. If I'm running a singaround, I make a mental note of who comes in when and I'll miss out the people who stand at the door then jockey for position to get the next spot then bugger off when they've sung - I'll let people have a go who have been waiting then come back to the Johnny-come-latelys after they've had a good listen. I also think it's a good idea to have a bit of banter with the punters - ask them their names then everybody knows who they are, how people are finding the festival, etc. It makes it go along and you're not just sat there on the singing Merry-go-Round. I find that the "jump in when you like" type singaround is a bit offputting when if like me you like to take a second or two to compose yourself before launching into song then find someone jumping in before you.
Favouritism (thanks for bringing that up Harry) pissed me off at a recent festival. I indicated that I was willing to do something when someone else declined but I got ignored by the organiser. I shall say no more about that.
Put me down for a "yes" vote on this one

Cheers
Mark