Wall Street Journal gadget guru Walt Mossberg, in a review of the new Internet-based Amazon and Google music services, offers his take on what's really needed for a great in-the-clouds music service:
I call these two new services rudimentary because they have a major pain point: Before you can use them, you must upload all your songs to the cloud service. Depending on your Internet connection (which is often much slower for uploading than downloading) and the size of your collection, this can take days.
By contrast, the Holy Grail of music lockers is one where no uploading is required. It's sometimes called "scan and match." Under this approach, the music service would first buy from the labels the rights to stream a huge catalog of music, and, with your permission, scan your computer to see which of those songs are present. Then, it would simply assign you the rights to stream those songs you already have via multiple devices of your choice, and even preserve your playlists of those songs.
-WSJ, May 19, 2011