The primary change is that app listings now offer the options to join or leave a beta test directly in the app, making it far more convenient for users. Once you're in a beta test, the listing in the Play Store will note whether it is a beta version of an app or even if it is a private unpublished version of an app that isn't available to the public. Unfortunately, some beta tests still require that you going a specific Google+ community (this option is up to the developer), so you would still have to do that before joining the beta from within the Play Store.
However, a possible change that would mitigate the need to join a G+ community is the option for users in a beta test to now leave comments for the developer directly in the Play Store. The UI looks just like a normal review for an app, but the comments for developers are only visible to the developer of the app.
source: Android Police