The fact that so many of these online services are basically using The Echo Nest to build their "music discovery" engines (aka their radio option) is a bit unnerving. What's there to differentiate them if they all use the same methods to pick what song they send you...?
Also shameless plug, I got really tired of Spotify, Pandora, last.fm robotic stations and built (http://www.ssradio.me). 100% NOT powered by Echo Nest.
It depends. EchoNest has a similar track feature for any given track, which could trivially be used for playlisting. But it also provides an incredible amount of data. There are countless ways you could create different playlisting algorithms with their APIs.
Also shameless plug, I got really tired of Spotify, Pandora, last.fm robotic stations and built (http://www.ssradio.me). 100% NOT powered by Echo Nest.