Version 5.4.1413N of Play Music brings the ability to start radio stations based on playlists that you created, a boon for those who have very particular tastes in music.
Previous, you could only start radio stations (playlists that are auto-generated based on tunes you listened to) from an artist or a specific song. Now you can create a playlist of different tunes that you like and click on the three-dot menu button to start a station based on it. For instance, Play Music creates an auto-playlist of all songs you thumbed up, so starting a radio station based on that should result in a selection of tunes that closely matches your tastes.
This is really the only visible change coming in version 5.4.1413N of Play Music, and the update should hit your device any time now. You can install Play Music or check for updates in the Play Store here.