A few recommendations for you that I also plan on for myself:
1. Keep the backend 'database' as simple (and portable) as possible. Files and folders is ideal, with metadata stored in the files (in this case probably as ID3,etc tags)
2. Have a simple API to the data so 'rich clients' could be developed that are capable of caching limited sets of data for offline usage.
3. Make the HTML interface somewhat skinnable so people with conflicting tastes to you don't get turned off just because of the style.
4. Focus on a great personal experience before adding lots of sharing features.
Good luck!