What do you think about something like a song "pool" or "current playlist", where you can just add (drag and drop or a new adding symbol) the songs you just want to listen to at the moment, but don't have to create a new playlist?
My mood changes a lot, so does the music I'm currently listen to.
So I wouldn't have always to create a new playlist according to my current mood.
(I already have too many playlists... It's getting confusing!)
It could be a bar on the right, just like where the friends activities are shown.
And if you want to, you could save the "current playlist" as a new playlist.
I think Windows Media Player had something like this. So maybe you can imagine what I mean.
Hope you can understand, what I tried to describe. 🙂