I'd like to be able to keep track of what I download, so that if I ever switch to a different service, I can re-aquire the music from that service. Is there a way to see a complete list of all my Xbox Music downloads, separate from my other music files?
The Xbox music files should have been downloaded to your My Music/Xbox Music folder. Any other music you had before is probably located directly in My Music folder.
With the Zune Desktop you could in the collection view choose a dropdown on the top left to view Zune Pass Content. This feature doesn't seem to be available on Xbox Music. But as mentioned the downloaded files should be in the Xbox Music folder.