I once fell off a treadmill because I was laughing at On the Buses :D
I've been a member of various gyms over the years, and I believe that friendly staff / trainers are the way forward. I once stropped out of an aerobics class in Germany when a cow of an instructor singled me out and said that I was uncoordinated :(
I like classes - the Les Mills ones are really popular (Body Pump, Body Step, RPM, Body Balance, Body Attack etc), and would definately be a good selling point. I wouldn't join a gym that didn't have Body Pump on the timetable.
Keep the classes fresh and change the timetable regularly - but try to avoid gimmicks like drums alive (hitting a swiss ball with a drum stick... hmmm... what kind of a workout is that??). If you think that I made this up, here's a link:
Providing towels is also a plus, as is free shower gel (in dispensers) in the showers. Am not too bothered about free body lotion and cotton buds.
Are PowerPlates popular in Poland? They're big here in London, but I get the impression that they're just a bit of a gimmick. I've used them a few times, but didn't really feel much benefit.