What I like:

Low/Dark Lighting
Good ventilation
Good music (especially when there's a theme like motown, christmas, a particular band) (I hate techno though)
Music that isn't too loud
An instructor that isn't too loud (hey, this is supposed to be fun, not give me a headache!)
Options (like speeding up, standing)
An instructor who doesn't expect everyone to be on the same page (some instructors give you a hard time when you tune them out)
Not too much time spent on high resistance (I personally like to spin out after doing really heavy resistance but some instructors don't give me enough recovery time)

Pet peeve: I only went to one of her classes, but this one instructor tried to motivate us to spin faster by saying "think of something that you're always in a hurry to finish". I'm in grad school, stressed to the max, and couldn't take it. I had to leave. I don't go to spin class to be reminded of all the things that stress me out - I go to forget about them.

Unless people stop coming, just keeping doing what you're doing. You can never please everyone.