For us, the fact that the swinging world is totally different to the real world - normal conventions, morality, whatever don't apply. Society thinks you should have sex in private with one other person that you have a relationship of some sort with, but on our first swinger club visit we saw playrooms full of people fucking without a care in the work, swapping partners, cumming, etc, while being watched by others. We soon moved from watching to joining in as it seemed so natural and joyful, and being watched didn't bother either of us.
That's still the main appeal, the fact it's all so 'unconventional' and not just clubbing either. It takes things to a different level when you have a drink with friends (nothing unusual there) but then you all go upstairs, undress, swap partners and fuck on the same bed - which certainly is unusual for most people. Most people wouldn't understand! And the way you meet complete strangers in clubs, find a playroom and do the same. Or just swap with another couple in a playroom, usually without saying a word. It's all pretty amazing really.