Google+ take more of a Twitter approach, allowing anyone to "follow" anyone, no matter what. In a sense, this puts Google Circles at a greater risk to putting Twitter out of business, as it takes the Twitter follow model and lists, and adds privacy settings to it, using those lists to make that happen. I bet we'll see Twitter do this in the near future as a response to Google Circles.
The biggest thing Google did right this time around is they did what no other social network was doing. They took privacy, and put it smack in the face of the user to make their own conscious decisions.