I was out with a friend at the weekend, and was able to observe his first attempts at choosing music.

He was used to queuing music with XMMS or something, so was continually being surprised by selecting a tune and having it replace the current playlist. IWBNI the default could (temporarily) be changed from Replace to Insert or Append, like there is for Search. (If implemented, IWBNI there was on-screen indication of what would happen).

He was sometimes surprised to select what looked like a playlist and have a track replace the current playlist. IWBNI there were some indication of which menu items are themselves playlists (like you get with submenus in most windowing systems). Perhaps tiny little down-arrows that appear when the item is selected?