iTunes 12.5, currently only available with the beta version of macOS Sierra, offers a number of small changes, and one of them is a Dislike flag you can set for a track. It’s binary, just like Loved: you either dislike a track or you don’t. And if you have loved a track then dislike it, the Loved flag gets removed. If you dislike a track then love it, the Disliked flag gets removed.
To dislike a track, right-click it and choose Dislike:
When you dislike a track, it displays with a broken heart:
Doug Adams also points out that dislike is a new track property for AppleScript, so you can create a script that gathers all your disliked tracks in a playlist. I would also expect this to be available in smart playlists, but it is not yet the case.
So why would you use the Dislike flag? Since you can mark an Apple Music track or playlist as something you don’t like, this helps the Apple Music algorithm improve recommendations. But you might also want to use it to flag tracks that have problems when you play them back. When I rip CDs, I don’t listen to them right away, and I occasionally find that there are problems with a specific track, such as diginoise or clicks. I rate these tracks one star so I can check them at a later date. You might want to use Dislike for this, or for tracks with no artwork, or tracks where you want to change metadata, and so on.