Even though I don’t participate in dance classes (I can’t dance to save my life), I believe the reasons are similar as to any other fitness classes. I would say all of the reasons you have mentioned are good enough when looking for a class. Everyone is different, so no one class or instructor would satisfy all of the participant’s needs. They come to a class for their own reasons (music, goals, instructor, social time, environment, intensity, location, fun factor, etc.), so being able to keep everyone happy it will be impossible. Personally, when I look for a class it has to do more with helping me with my goals and I always look at the experience and credentials of the instructor.
I think that dance fitness programs present the unique challenge of uniting fun and safety. Music moves us, quite literally, and that is a beautiful thing. But it also can cause us to over-write our common sense of what is a safe move for us.
From that perspective, I would expect an instructor to make safety a priority while still making it fun.