A lot machines to be continuously "tightened up". For example the pec-deck/rear delt machines. The bolts have to be tightened or they get wobbly and loose.
The paper towel machines need to have paper towels in them and not be jammed up (the automatic ones seem to break down a lot) and the spray bottles need to have something in them; otherwise people won't wipe clean their machines off when they're done using them. Lifetime has gym wipes, which are awesome but probably not cost effective.
For some godforsaken reason, people like to tie knots in the attachments, which ruins them and looks like shit. I would ban anyone I caught doing this for life.
You want to have lots of 45 pound plates and lots of places to rack them. I would have very few (if any) 35s.
If possible, have the Smith machines close to the dumbbells. That way people aren't dragging the adjustable incline benches back and forth across the gym.
Don't let people leave giant duffel bags and backpacks all over the gym floor. It's a tripping hazard and it makes the gym look like shit.
Personally I LOVE a gym with no music. If you are playing music, it shouldn't be so loud that I can hear it with my headphones on.
Of possible, double up on the machines that get used the most (pec-deck, preacher curl machine, hip-thrust, etc) and get rid of the machines that people don't use much.
Cables, cables, cables. The more cable stations and attachments you have the better. You get maximum versatility for your space with cable stations.
Don't have a punching bag. It attracts too many douchebags that want to pretend they're UFC fighters.