Just a couple suggestions to make the rating system even more useful.
I'd like to see a command that toggles rating overlays on thumbnails (if there isn't one already?). It would go nice in a button that also toggles the rating column.
More importantly, it would be great if folder ratings could display the average rating of the files inside. This would, for example, give you a good indication of what your favorite album is if you have the thumbnail overlays turned on
And lastly, the built-in image viewer could definitely use some rating buttons in the toolbar.
A command to toggle rating overlays makes sense. There are similar ones for dimension bars and thumbnail labels, but not for this yet as far as I can see.
A configurable viewer toolbar would be nice in general, although won't happen in the very near future.
I'm not sure about showing average ratings on folders. That could take a long time to calculate (especially if fully recursive or just with a large number of files), folders can already have ratings set on them explicitly, and to me at least the average rating of files below a folder doesn't always tie up with the rating for the folder itself (thinking of the album example, and "filler" tracks).