Jeremy Schlosberg shows some good insight into the failure of the current playlist ecosystem, in his article Playlist Nation, which was posted to the pho list.
As something of a playlist nut myself, I recognised a lot of truth in his assertions about the personal, internal use of music. He's quite right, for instance, that it's often more fun to make a mixtape for someone than to listen to one they made for you. But I disagree with his main point that playlist sharing has no future.
For a start, he has it backwards about Apple, whom he credits with inventing the playlist, and putting it centre stage on the iPod. Apple's approach to listening is: music all the time. Party Shuffle is the epitome of this - you can add music if you want, but in any case it will just add random stuff to keep it going. And generally - he's right here - that's all people care about. But it's the antithesis of playlist making, which is about a journey from A to B via X, L and P. The functionality iTunes and the iPod lack is to turn the bunch of songs you happen to have just been listening to into an instant playlist, effortlessly. You have to think to yourself: "today I'll make a playlist". Which of course, unless you're a saddo like me, you rarely do.
It's also painfully obvious that the author hasn't has first-hand experience of Spotify. It's a game changer with regard to playlists. He's wrong about it not having playlist sharing built in, but more importantly playlists are its default use case. There is no Party Shuffle; it wouldn't be very helpful anyway as you'd be shuffling the whole of (available) music.
Spotify may look like iTunes, but functionally it's more like other music players such as Winamp or foobar2000, where the only distinction between "Now Playing" and a new playlist is whether you've hit the 'Save' button. Spotify's 'radio stations' are awful, so the user is confronted with two options: either create a playlist or listen to someone else's. It has been a boon to magazine sites and other tastemakers, such as Drowned In Sound or misterchristrout.com, which publish regular Spotify playlists.
I take Schlosberg's general point about playlist sharing. It can't be anonymized; can't go many-to-many. I used 8tracks a couple of times then forgot it existed because it's too hard to find the nuggets. Whereas I often listen to my last.fm friends' libraries, even though they lack that "journey" quality I crave.
But I think playlists have yet to take their proper place in music discovery because there are still unnecessary hurdles in their creation, for which Apple is partly to blame. Spotify has forced users to at least think about finding, creating and sharing mixes. Also, currently portability is a real headache, and the cure for this, the XSPF format, has yet to be properly realised. But the cloud will smooth this problem, even if the wished-for global music id system fails to emerge.
My playlists are primarily missives to my friends and future self. I want them in a permanent form, available everywhere. Combine a properly-stocked Spotify with a facebook-like intimacy and you might get an explosion of one-to-one events.