Comments on apps is a good example, but it’s a small one.

The interesting data held on Google Play includes the list of developers and users and their credentials (passwords). Every user must log in using their Google account in order to access the service. If Google Inc where to ban them from the service, they would be able to do so.

In addition, every developer has the list of apps they’re selling. So the apps themselves and their data (title, description) are also relevant.

The entire sale mechanism is also relevant. For a user to buy a paid app, they would need to pay for it, so this transaction needs to be stored somewhere as well as proof of purchase. Apps also support a mechanism of in-app purchase, so these would need to be recorded somewhere as well.

Every user has the list of apps that they’ve downloaded. They are able to download them again or automatically download updates.

Reviews are also interesting. The rating each app receives influences its position in search results, so the search order and categorization of apps is also important.

As you can see, there’s lots of interesting data that currently is the sole property of Google Inc.