That's actually a good example though. I can give it a precise incantation to play a particular album, playlist, channel, or artist and it works (mostly). I can't in general tell it to play some "soothing jazz."
You can tell Google Now, Cortana, or Siri to play jazz. Siri recognizes "smooth jazz" (And Google might, but it froze up on me). Curating moods of music is, so far, kind of a niche thing that is mostly left to humans like Pandora's music genome project, or Apple's Music service.
I guess I picked a bad example :-) On the other hand, you do either have to pick from pretty broad categories or have to go with a specific list that you or someone else has curated. But it's a hard problem.
