Regarding your email is your identity.. I think, you're identity is your identity. Email Twitter Facebook these are best considered - not identities but means of verifying your identity. So your "identitly record" in a system is related to each of those, but not one of those defines it. For a long time I thought using email address as your de-facto identifier as a login name made good sense.
I'm an older guy (w/ a teenage daughter), and its striking to the extent email is becoming much less relevant to the younger generation. They will inevitably have all of Twitter, Tumbler, Email address, and mobile phone number, but keying in on one as the God "identifier" if you will feels a little off.
The box with login w/Twitter, Facebook, etc.. seems the right solution for the present, but still feels not quite right, at least not totally elegant. A universal standard for internet identification of course would consolidate and simplify things, but not just the adoption by so many providers, but the concerns about privacy and tracking etc.. would seem difficult to even get off the ground.