According to Kate Cushing Product Manager for Google+ Hangouts, through G+ new hangout for iOS in iPhone, iPad, and iPod Touch users can have free calls to the US and Canada.
There are more features available to them who are not calling US or Canada, as they can make and receive calls from any of your iOS devices through Google Voice. Additional features include send animated GIFs in the chat system, incoming messages will no longer stop music.
Cushing focused mainly on the features according to the users feedback, rest all have been done to make app more simpler, useful and better.