I wasn't so keen on this feature as I thought even though you could play, it would have been mostly automatic loops, though that isn't the case it would seem.
From what that video says, you actually have full control over notes, etc, by selecting ranges of a guitar, so you can actually create what ever you want, with the notes that you want, and create an online band in a sense to get known.
Plus, this means looads of music for everyone since the downloads will be free.
I quite like that. Imagine recreating some game music yourself and getting it up there (copyright infringement?