While the HN community understood that, my mother was never going to grasp the idea of ownership of a URL as a login credentials. Many people don't even understand URLs and just Google for everything they want.

Email addresses have been used as login credentials for a while, so people are used to that. The explanation of Persona might be a little weird, but it's not as totally disabling to a nontechnical user as being asked to sign in with OpenID.