Lots of free weights, roomy, clean.
Weights, machines, and cardio equipment all well-maintained and operational.
Good location, hopefully not too crowded during non-peak hours. Nice to see a few hot women around as well, but that's not a requirement.
And the staff, management, and owners need to know that the customer is king. I left one gym many years ago because I didn't like the way the owner's son spoke to me one day when I had the nerve to interrupt his phone conversation to try to give him my monthly gym dues and request a receipt. I kept the money and walked out and never returned.