pretty much what anne_77 sez:

Low/Dark Lighting
Air
Good music not too loud
An instructor that isn't too loud or tries too hard to be funny or cheery.
My favorite spin instructor left. Her philosophy is "same in spin as on a bike". Form, fit, hill climbing, it all built cycling skills. She taught me what little I know about HRM, that is what I didn't learn here. Best of all she told people to "get outside and ride". Current crop of instructors are ok, but none as good as her.

"Pet peeve:...one instructor tried to motivate us to spin faster by saying "think of something that you're always in a hurry to finish".... I don't go to spin class to be reminded of all the things that stress me out - I go to forget about them."

Right, above and aformentioned spin intructor was fond of saying "this is your time to relaxe and do something for yourself". Just like a ride, except indoors, going nowhere, with posers ;-)

What I want in a spin class is it should make me feel like I'm gonna die, should be very spent after. I like it when i can barely walk out.