Having some PKI background (I'm recovering....) I know these are the questions upon which identity systems can founder.
4. What is the threat model?
What threats is OpenID designed to protect against? Accidental failures at a participating party? Malicious behavior by users? Malicious behavior by relying parties? Malicious behavior by OpenID providers? Wiretappers? Hackers attempting to penetrate a relying party? Hackers attempting to penetrate a provider? Hackers attempting to penetrate a client system? Cryptanalysts?