Well, I went to a fabulous party on the weekend. For a start, it was for a pretty spesh bloke who's managed to accumulate a number of diverse and interesting friends (which ticks the first box of party must-haves). Just like a dinner party should have an eclectic bunch of people gathered, so should a party. Too much similarity can lead to boredom, and a boring party. I met some amazing new people, caught up with old friends, and learned to love some people I'd admired on fleeting acquaintance.
Secondly, the location was perfection. The right size space, decent moody, yet not-too-dim lighting, and sections where people could sit, stand or feel inspired mingle. That's why a party in a large, open space doesn't work. A party is like a garden, you shouldn't be able to see everything at first glance. You need to be led around, with surprises and delights around corners.
Decent alcohol is, of course, a must, and finally - really good music. Music makes the party. Music needs to enhance your mood, move things forward and every now and again cause you to squeal excitedly "Oooh, I love this song!" and to do a little dance on the spot. When you have a party that people will continue to talk about for at least as long as you've been on the earth, you know you've had a hit.