This project is born out of a frustration I had managing my music library. I have found playlists, genres, and other ways of organizing my music too restrictive and cumbersome to keep up with. Here are the main features I want out of music player, in order of priority:

Web-based interface.
Mobile interface (could implement the Subsonic API on the back-end to be able to use pre-made mobile apps)
Song tagging.
Playlists.
Read meta-data from ID3.
Transcoding on the fly.
Read songs from Amazon S3.
Stream from Amazon Cloudfront.
Bookmarks (temporary song tag marking location in a playlist/album)

I have not found a music manager that gives me all of the above, so I'm going to write my own.