http://www.forbes.com/sites/kashmirhill/2011/08/29/googles-e...
http://blogoscoped.com/forum/164385.html
http://www.computerworld.com/article/2517360/security0/repor...
The "G+ is just an identity layer on top of Google" was a common soundbite reported by executives at the time, and perhaps they even believed it, but my engineers-level view as someone who worked with the G+ codebase was that this was mostly marketing speak to differentiate us from Facebook and deflect criticism of "Well, if G+ is a new social network, why would I use it over Facebook?" By saying it's the identity layer for Google, they can then say "Well, you use Google already, this is just the social layer that ties all Google products together." On a technical level a "G+ account" was just a bit set in your Gaia record that indicated whether you had opted in to G+ features.
When they launched all the talk was focused on the social network. The focus should have been on the primary product which is that they introduced an account system. The social network just simply came with it for free. You could use it or not use it.
Google accounts meant you no longer needed to use your Gmail accounts to sign up for Google services.
Could bind everything, Gmail, any other Google services all to the same Google account. When Google then pushed to get people onto the new accounts system people thought Google was trying to shove the social network down their throats.
The social network again, though was secondary to the account system. Google+ likely would have been better received had people been properly informed on the benefits of creating a Google account.