Don't we all have them. I can tell you one of mine.
Dancers who wear a jingly jangly hip scarf to a workshop. I have experienced two kinds of workshop facilities. One is LARGE and that makes it hard for people in the back to hear, and the other is SMALL and this makes the acoustics horrible. Both of these problems are only exacerbated by the noise of a coin scarf.
I found a great post here about basic workshop etiquette.
What is your pet peeve when it comes to dance?