I didn't like the sound of this at first, but thinking about genres and moods, it's actually quite a nice idea.
Here's a website that uses a tag-cloud interface to create playlists:
I think songbird has a tag cloud plugin, but none of the other popular music library software seems to have this feature.
Out of interest, what variable would make the 'rock' tag larger than 'trippy'? For instance, would it depend on the amount of those tracks in the library? Perhaps the sum of the play count? Maybe the sum of the ratings? What would be most useful?