1. Mobile Advantage: Google is making use of its mobile operating system by allowing instant-app install to Android devices when one connects with Google+. For instance, when Android user signs in with Google on a partner's website, he or she will have the chance to instantly install that app to their mobile device, even if visiting from a desktop computer.
2. Call-To Action Buttons: Google launched "Interactive Posts": within Google+, partners can have a prominent button display with a short call to action displayed alongside content. In fact, Google has launched with 100 possible buttons partners can utilize-- a direct challenge to Facebook's, ubiquitous 'Like.'
3. Other Google Products: This could potentially and easily tap-in to a number of Google's other products, such as Search, Gmail, Maps, YouTube, and so on. An example from HuffPo: "When you find a movie you want to see in Flixster… and you're signed into Google, you can one-tap add it to your Google Calendar." In other words, signing in with Google could bring the entire Google ecosystem to other services.
4. Selective Sharing Only: Google says it won't let apps "frictionlessly share" updates. Facebook's app integration has infamously lead to update spam to without any user action involved, drawing privacy concerns and complaints from users. Google plans to keep such updates from being a problem.
However, it's important to remember that Facebook still has one big advantage over Google. As Josh Constine wrote for TechCrunch, "Most people do not have richly filled-out Google+ profiles with data to personalize apps they sign in to."
Still, Google+ Sign-In represents a significant step forward for Google and its potential for growth.