The most noticeable change is the addition of a back button to the lock screen and notification navigation controls. This way you can repeat a song or cycle back to a previous track that you want to hear again.
The full-size navigation drawer is here, which is the new standard for Google’s own apps. It features your Google+ profile pic and background image, along with key navigation options.
Google also added in bio and background information to each artist’s page, bringing it up to date with what you see on the desktop version of the Play Music app.
Also, when you scroll the entire header and tabs now hide, giving you more onscreen real estate. To get them back, just start scrolling back towards the top. There are other touch-ups sprinkled throughout, which help make the Play Music app feel smoother and more consistent. Check it out yourself to see what's the latest by looking for the update on your device or grabbing the APK if you're impatient.
The impact on you at home: These are some good tweaks that should make your Play Music experience better—especially if you're a subscriber. It’s also another indicator that fragmentation isn’t necessarily that big of a deal as it seems. Google pushes updates to its core apps and services directly through the Play Store so the carriers can’t keep you from getting the latest and greatest experiences.