Something that doesn't require you to be a techie to understand. OpenID is a bit advanced for many users.
FB Connect actually does a pretty good job at being "easy to use". Just log into your FB account and you are set. I know it's not fair, but most people havn't posted anything to that openid website.
Google or Yahoo logins would work, since there's a recognizable brand name and there's a good chance that the user has an account on those sites.