People want the value of reputation without the cost of exposing their person information.
Facebook comments forces you to expose personal information. I want to leave comments with a reputable identity, just not linked to my real one.
Here's how it could work: There could be metatags that specify where to submit the public key. The website will then send back a message that can only be decrypted with the private key. The browser has the user type in the password to access their private key, decrypts the message, and sends it back to the website. The user has now started a session with their public key.
Since you would use the same public key on all websites, your identity would be the same. This would not require a central server like OpenID.
The 10% you miss are cleared cookies and different browsers/private browsing. This could be seen as a feature however since extra anonymity is gained through existing means.