i suppose it's like any kind of party, very hard to put your finger on it. obviously the location, people and mood but maybe it's something more like the right alignment of the stars! or the feng shui!!!
seriously, i think having clear rules is important in play parties, so people feel like they can relax and yet not have their space violated until they are ready. people come for different reasons, some come knowing they will only watch and others come looking for action and the rest undecided. my impression is that it takes a longer amount of time for people to relax enough to actually engage and that should be taken into account during the planning.
makes me wish i had plans to go to one....i could report back!