Like Facebook Connect, Google+ Sign-In is a single login method that can be used for websites and mobile apps that at the same time "imports" some of an end user's social profile information. Naturally, it will be Google+ profile information, not Facebook information.
Twitter also has universal login functionality.
Those who login using Google+ have the same sort of functionality they have with other universal login procedures. They can share all the activity and information through Google+ with selected circles, specific people, or no one else at all. In addition -- and as a plus over other login services -- Google+ Sign-In supports Google's two-factor authentication functionality.
Looking at alternatives in the third-party login space, Facebook has offered Facebook Connect for a little more than four years already, and Twitter has permitted universal sign-ins since 2009. Even LinkedIn offers a similar sign-in option.
The fact that Google+ trails in the login space could be an issue. It's not as though everyone in the world was demanding another method to use to login anywhere. And right now, Google+ Sign-In isn't exactly ubiquitous. The service has started with 10 services: Flixter, Banjo, the Fancy, Shazam, USA Today, FitBit, and others.
Adoption by other sites and apps, too, will be a key.