Thanks to iOS 8, you can tell Siri to rate the song currently playing in Music from one to five stars. This is useful if you also manage your music in iTunes on a Mac or PC, since song ratings can power smart playlists (for example, you can create a playlist of all your music rated four stars and above, or just your five-star songs). But as of September 2014 and iOS 8, Apple has still not added smart playlist abilities to iOS.
[thanks Tyler!]
