That is right. But till now, I could use a simple Google acount (not G+) to login for example at Stackoverflow.com. In future the button will change from "G"-L

That is right. But till now, I could use a simple Google acount (not G+) to login for example at Stackoverflow.com. In future the button will change from "G"-Login to "G+"-Login. As somebody else stated, Google said, that a normal Google account will suffice in the future, but alone the wording and the new button will alienate people.