Google announced several improvements to Google+ Sign-in today.
A major one is support for all Google account types. Now, Google Apps users and even those without Google+ accounts can use the feature to sign into third party apps and websites.
The second upgrade is easy migration from other auth methods.
As product manager Yaniv Yaakubovich, explains, "If you’re using OpenID v2 or OAuth 2.0 Login for authentication and want to upgrade to Google+ Sign-In, we’ve made it easy to do so; it’s entirely your choice. Google+ Sign-In can grow your audience in multiple ways — including over-the-air installs, interactive posts, and cross-device sign-in — and now it’s fully compatible with the OpenID Connect standard."
There's a migration guide here.
Finally, they've added incremental auth, which is dsecribed as "a new way to ask users for the right permission scopes at the right time, versus all permissions at once."
"For example: If your app allows users to save music playlists to Google Drive, you can ask for basic profile info at startup, and only ask for Google Drive permissions when they’re ready to save their first mix," explains Yaakubovich. "Likewise: you can ask for Google Calendar permissions only when users RSVP to an event, and so on."
Google recommends developers ask for the minimum set of permissions, and then for more as required.