I'm interested to hear about your hopes for Google music, and the kind of features you are/would be expecting. I'm asking because currently working on a similar service :)
Also, it doesn't allow me to access cloud copies of music, only my own library. Honestly, the concept of an "own library" is obsolete. I want a service that lets me stream anything I want (like grooveshark), with a powerful and intuitive interface (like old iTunes), and a "local library" feature (like Spotify) of anything I want for offline listening. Add in a worthwhile adaptive radio feature (like Pandora) with sharing capabilities (like last.fm) and you've got the whole package.