1.) Guest who don't dance and complain
2.) Rambunctious kids if invited or young children who need to head home and parents have to leave etc.
3.) Open bar (guest hover around the bar vs dancefloor)
4.) Poor DJ who can't mix and read the room, staying in songs too long
5.) if you're invite list is 25% people who aren't social and 25% elderly or with young child, 1/2 your guests won't dance