I don't know if this is the case for other providers, but with a blogger.com OpenID, I need to enter my blog URL as an ID, then I get a prompt asking me if I want to always tell the site I own my OpenID, then I get asked for my username and password.
Why in the world can't I just have the OpenID and the password fields to deal with and why do I possibly need to go through 3 screens just to post a blog comment? (having to log-in to post a comment is pretty lame on its own but I digress :P)
I can understand sending the user to the blogger.com site for the actual login over https, but can't it figure out my username from my OpenID? I mean, it's a one-to-one relationship and usernames aren't exactly secret.
If OpenID is to become popular, it really needs to be as easy as logging into 99.9% of login-based websites.