Anonymous mode is an interesting idea. As for metadata & settings etc., these could also be stored on the IPFS, right? IPNS, to be precise; you just need an IPNS publishing key shared between all of the user's devices, then every device can update them to whatever the latest setting etc. is.
A public mode could also be nice, e.g. if you have songs of your own you want to make publicly available; then all you need is a website that gathers the IPNS hashes of people's public libraries (or they register the IPNS hashes there), with player functionality, social stuff etc., for a kind of distributed SoundCloud.
And, of course, a system for sharing parts of a library (or even the full one) with friends & family.
Don't know how to do the authorization thing. Maybe you can even use IPNS keys as a system to authorize users and assign access to certain content?