I can't just pick one and pretend the others don't matter.
Seeing and being seen is an aspect, but not a too big one. On the one hand, meeting people can be nice, especially people whom you've always wanted to meet in-world. You may know the feeling that the Hypergrid seems pretty small when you're at a party with a dozen famous avatars around you. On the other hand, I don't want my friends list to become too cluttered with clingy chance encounters and/or people who live in a timezone too far from mine.
Dancing is definitely important. In fact, instead of just plopping down on a line dancing floor with everyone else or putting the Clubmaster on auto, I prefer to switch to a dance that fits the song. I prefer couple dances because the omnipresent Clubmasters are vastly better in that field than when it comes to solo dancing. Good thing I have a sister now. (Kudos to Cayoun and his menu-driven dance pads, by the way.)
How important the music is depends on the music. There are events where I only go to meet people I already know because the music isn't my cup of tea. And then there are events where I go for the music itself. Of course, there are also events which I attend for both.
Dressing up for an event never becomes much more important than the event. But if there's a dress code or only just a motto, it's really fun to dress the part, and I take this quite seriously to the point that I hardly go to events I can't dress sufficiently appropriately for. And this can get difficult with a mesh body for which not a single piece of garment has ever been fitted. In fact, beach is more difficult than 80s, blues, reggae, ballroom, movie themes* or Dead Musicians' Society. Nonetheless, I do have quite a selection of motto outfits.
*actually make me want to rez a changing-room next to myself