I think this is an an awesome product but I just had an idea which I always feel would be implemented but as the days pass I do don't see it.
I feel like the friend stream takes up a fairly large portion of the interface real estate but is the most under-utilized area.
It's great to see what my friends play but so much more can be built off this section to make it more engaging and interactive.
It would be awesome to "like" (internally through Spotfiy or through the Facebook API) a song that my friend plays. As multiple people like a specific song or artist, it should show or give an option to show who else likes a specific song and/or artist so that I can have a good sense of which of my friends or group of friends like the same songs or artist. This would make it more likely for me to take the time to listen to a particular artist/song as well as allow for the person who played the song on the feed to see that I liked the song they played and get friend-support. Once you like a particular song, it would be permenantly associated to that song (so you don't have to like it multiple times). I'm sure this could also help with the radio product.
On the biz end, artists/spotify can use or sell this info to see what their fan-base enjoys.
Just an idea I had where I think multiple parties can benefit.
Although this idea has been suggested first, i'd like to point you to a similar, more recent idea here: