One of the most popular features of Google+ is the Hangouts group video chat component. It's a convenient way to easily setup a group meeting whether for a casual chat or for business video conferencing. It also works smoothly even with larger groups and is completely free. Plus, Google plans to open up the app to allow third-party apps to work with the feature.
Google Real-time Communications Tech Lead Justin Uberti posted on his blog that they intend on publishing the specs needed to interoperate with Hangouts. This means that third-party developers will be able to create apps such as mobile clients.
Hangouts is already a worthy competitor to Skype, and doesn't require a premium subscription service. Moreover, Hangouts also doesn't require a dedicated app. It functions directly in-browser with Google already working on ensuring its compatibility across many browsers without the need for downloading plugins. Adding the ability to integrate with third party apps could really push Hangouts ahead.
However, Google+ rival Facebook could be deeply integrating Skype's video chat service, if rumors for a press event announcement this week are true. It will be interesting to see how all this plays out in the coming months, especially when Google+ reaches its public debut---believed to be July 31.