I've noticed for a while that I'm much more likely to skip over a good song on my iPod than I would if flicking through channels on the radio.
Thinking about it, I wondered if there's a certain loss of enjoyment in a song when you can hear it on demand.
But then on a trans-Atlantic flight on Monday, I noticed a similar phenomenon with playing my entire library versus just the playlist of my most highly rated. I'm far more tolerant of, say, a three-star song when listening to my entire library than a four-star song on my highly rated playlist.
So I've come up with the following formulation: The chance of skipping over a song is strongly positively correlated with the chance that the next song is at least as good.
With a "highly rated" playlist, there's a good chance the next song will be as least as good as the one I'm listening to, so I'm more inclined to skip the current song, even if I really like it.
With my entire library, if I'm listening to an "okay" song, there's a reasonable chance the next song will be one I like less, so I'm more inclined to enjoy the current song.
That's my current hypothesis, anyway. I haven't read George Zipf's work but I wonder if it relates to this.