Pretty sure Spotify's Discover Weekly works specifically because of the constraint they set up for themselves. It's updated once a week, and there is a finite number of songs in the list. With other services, people turn on the "recommended" station and expect an infinite stream of music that they will like.
Last weekend Amber, Anima and I drove from San Diego to Palm Springs to YxYY. We were streaming music from Spotify on the whole drive, and Spotify was scrobbling to my Last.fm account. Of course I was also tracking my location like I always do. Then it dawned on me that I could put together a map showing all the songs we listened to and where we were for each! Below is the result!