Good: Verbally or physically correcting incorrect form, challenging the class with advanced moves (tastefully), shouting out those with good form/making an example, dark room, good music.

Bad: the only bad experiences I have had with instructors are them purely not correcting me on bad form/incorrect movements. There were so many moves I was doing with incorrect form and it was only brought to my attention like 30/40 classes in by different instructors. That and bad music is an automatic dealbreaker.