I'm gonna stick my neck out here and say, what I look for is variety. My taste in salsa music is very eclectic, and although I do like medium tempo salsa, there are also many faster songs I love dancing to. Some of my favourite songs are classics while others are very new and "hip".
A varied set allows you to try different styles of dancing with different leaders. With some leaders I love dancing to sensual mid-tempo classics, but there are also leaders who are great fun to dance turbo salsas with, so I want both. Some songs are made for dancing with Cuban style dancers, while others are great for LA style. What I don't like is the same sort of stuff throughout the night -- a night that doesn't go above the 2nd gear is just as disappointing as getting too many turbo salsas and wild descargas.
Tonight is my favourite salsa night, and one of the things I like about it is the DJs. The team is very good at reading the dancefloor and always comes up with a varied and interesting set.