A program that let's me tag my music a la flickr or del.icio.us, so that I can create tag based playlists.
iTunes only offers five stars ( an arbitrary rating axis) and a genre for each song.
iTunes 6 seems to let you select individual songs and let you add a "do not add to playlist" tag. Unfortunately, you can not add this tag to multiple tracks at once.
Do I really want my guilty pleasure tracks showing up on a party shuffle playlist?
Also, private/protected tags would also be nice.