Google highlights great design in Play Store apps
Given that Android L is on the way with its Material Design makeover, you would expect this collection to be filled with apps that have already adopted the new design language, and that is what you'll find. But, Google is also highlighting this group of apps for specific design details within each. For example, Google talks about the "beautiful colors and friendly illustrations" in the Lumosity app, the "tasteful typography" of Spotify, and the "delightful animations" of the Yahoo News Digest app.
It is a very nice set of 12 apps, and the vast majority are free. Only FlightTrack 5 and Runtastic Pro are paid apps, and each of those will set you back $4.99, although you could get the same "playful palette & intuitive gestures" in the free version of Runtastic if you prefer.