Users are comfortable using an e-mail address as an identity. Everyone has an e-mail address. This makes sense to end-users.
E-mail addresses are unique and they accurately identify who someone is. I have no idea why this wasn't the standard in the first place.
If every service simply required an email address, and the standard provided for a way to find out the access point from the OpenID provider, then we'd be all set and everyone would be able to implement this change.
The URL is simply a pointer to my identity, which is all I should have to give. The different formats the providers use is what makes it confusing (example.com, username.example.com, example.com/username, and now [email protected])
If the libraries don't already support the "@" syntax, they should.
If Google's way of doing things requires any proprietary changes, I'd say it's unacceptable, but if that's the only change necessary then it seems like a reasonable way of doing things.
The reality is that users don't know URLs as identification mechanisms. They do know email addresses.