To keep the player's real name secret, a change is being made to Player IDs. In the future, existing Google Play Game users will still employ their Google+ ID to sign-in, but new players will get a separate Player ID that is unrelated to their Google+ account. As a result, those using Google Play Games no longer will have their account automatically upgraded to Google+. Still, developers are being told that if their app uses specific Google+ features "such as requiring access to the player’s real-world Google+ social graph," new users will still have to have a G+ profile to play the game.
For most Google Play Game users, there won't be much of a difference. If you're a developer who writes apps for the Google Play Store, you just might want to take a look at the Developers Blog by clicking on the sourcelink.
source: AndroidDevelopersBlog via AndroidCentral