I'm at Waikato Uni, working on a research project regarding playlists--with a goal of designing good software to support playlist creation. First step, though, is getting feedback on what makes playlist good or bad.

So: what factors come into play when you see or hear a playlist and think, 'wow, that's good!' ?

Granted, the songs themselves have to be good, or the playlist isn't listenable. But after that, is there something special to the order of the songs? The theme? How long should a playlist be, anyway?

Tell me stories--I'd love to hear about particularly nice examples of playlists.