1) Will users remember an URL? Users already have enough problems remembering their own email addresses! And I don't think the "users are familiar with URLs" assumption is true: I've seen over and over non-tech users actually using Google as an address locator (that is, typing "Yahoo Mail" on the Google search box to access Yahoo Mail).
2) The phishing issue is a show-stopper, and convincing users to download a plug-in or install a client cert is simply not feasible. I don't see how they will fix this issue, unless if they convince Firefox and IE to provide an out-of-the-box fix (like a pre-installed plug-in or an open id CA cert)
3) It's not like user management is a huge pain from an implementation standpoint. And right now the risks of outsourcing user management to OpenID is higher than doing it in-house.