I'll launch a web app that will let users discover music on basis of its emotional aspect(for example, sad songs, patriotic songs) and will let them create playlist and share it with friends on facebook.
I've longed for someone to do it... something akin to StereoMood with a little bit of TurnTable.fm activity to it.
My app will have a global song database with emo-tags(user generated) attached to songs. It will let user maintain their own playlist, import songs from other public playlists or site's global playlists.
I always want something me and my music nerd friends can play with and share. Like keep a playlist going in the office but let any people add to it.
Google Docs for playlists?