Having playlists shared between music players and between machines would be a great idea; make a playlist on Banshee on one Ubuntu machine and it also shows up in Rhythmbox, and Banshee on your netbook, and Ubuntu One music streaming. A standard format for playlists should be defined (m3u? pls? Something else?) and details of where those playlists should be stored, how they should be synced, and how music players can automatically read and write them seamlessly.