Some random thoughts on playlists for music players (e.g. mp3 players). All the players I've seen use the song as the basic entity (which is fine) but have very weak notions of anything else. A playlist is often just a list of songs, sometimes with a name. Albums don't exist except as a common value found in the "album" field of some songs. I'd like to store name, author, rating, genres, and other metadata for my playlists. Once we can do that, we can get to the bit that got me thinking about this in the first place:
Albums are vendor-provided playlists. They're often composed of never-before released (or remixed or remastered) material, but that's a separate issue.
Also, the song entity that players toss around has more than one meaning, and they get conflated. One is as something that gets entered into a playlist. Another is as an audio recording. A song-on-the-playlist can be thought of as a playlist containing the single audio recording that it names. Thus, songs, albums, and playlists could all be considered equivalent units from the playlist's perspective, even though songs are the only things that can directly reference actual audio. The point of that is that they have a lot of types of metadata in common. Playlists have authors, they have genre, they have dynamic range, they have tempo variance, they have creation dates, and they could have user ratings. Songs have a few other things too, but they shouldn't have a monopoly on all metadata.
It's fun to determine patterns of behavior by looking at their side effects. My keyboard is all shiny and polished in the spots it gets the most wear. It seems I only use the space bar with my right hand, and only use shift, alt, and control with my left. Also, my touch typing is fairly apparent, with lots of wear on the home keys, as is the fact that I use a dvorak layout.