Quality of music is key for dancing and ambience. I don't like parties unless I get to dance.
Arrangement of space for dancing and socializing is also important... to not have people hiding in the corners and sides of the room, the spatial arrangement (furniture, etc) must invite them to sit down, have a drink, or stand all over the room and get to know new people instead of huddling.
I'd say lighting is also key.. keeping it low enough to be intimate, but not too low so that it feels like a seedy bar. I like putting up a string of lights or two (often just one color), and lighting several candles around the room. Groups of white tealights do wonder to create an intimate setting.
Yeah, I'm afraid parties just aren't what they used to be, now that I've been in a steady relationship.
But I'm not complaining... I enjoy retaining my memory of nights out and not getting shit-faced anymore.
As a couple we almost always attend parties with a good mix of both couples and singles, which I think enhances the social aspect of the party. Personally though, I don't enjoy going to parties where I only know maybe one or two people. It helps when I have a good foundation of people from which to go forth and meet others.