OpenID 1.0 and OpenID 2.0 both support a thing called "simple registration" (this is before the OAuth work) which lets the site you are logging in to ask the OpenID provider for some basic profile information - email address, nickname, postal code etc - which can then be used to pre-fill the signup form on the site. That's the feature that's meant to make it easier to sign up for new sites, and it's been working for several years now.