What is OpenID for? (Heavy criticism of OpenID)

As far as I can tell, OpenID is intended to provide a decentralized replacement for the current practice of everyone having separate usernames and passwords for each Web service they subscribe to.

On the one hand, I think this is a Good Thing, because the current way of doing things is so unwieldy and insecure.

On the other hand, OpenID's improvement over the current system is not very large.