For me i like the following:
Good music with a good mix (enough bachata, and some cha cha), and different styles of salsa.
A DJ that feels the crowd. (metaphorically speaking)
Lots of people to dance with, but not too many, i get wrecked sometimes when there arent enough men.
A good floor like the one in Dun na Si, or the Greville arms. Greville is nice and fast, i can pull off 6,7 sometimes 8 spins pretty easy. Dun na si floor is nice and big and smooth. Some floors wreck my shoes, or arent even and cause stumbles etc.
A good suply of shirts and a towel 
Air con! Oh a nice cool breeze make it sweeter!
And all the tap water i can drink!
The combination of form and expectation is important too, you know how sometimes you feel great, and are expecting a great night, and it turns out to be a disappointment, or sometimes when you feel cr*p and have zero expectations you go and have a great night!
Most importantly, meeting my friends. I love my friends, that why they are my friends
and for many of them a salsa night is the only time i see them.