I went back and looked at GPM  again to see the new instant mix and recommendation features.  And I'm fairly unimpressed with these compared with last.fm. I'm definitely getting the impression that there's a hefty dose of commercial push behind the algorithms. How else can I explain my library being exclusively dance and club music but having Gospel recommended to me?

Last.fm did a really good job at this stuff with radio stations/mixes based on
- Music like this artist
- Music like this genre tag
- Music like the stuff I scrobble
- Music my friends like
- Music that people who listen to this also listen to
And it was largely built wiki style by audience participation and curation. Such a shame that it's slowly fading into obscurity because 
CBS don't know what to do with it.

Google has a lot of knowledge and experience in applying big data algorithms to user participation. I would hope they'd put more of that into GPM and Music Key.