One of the big problems when trying to convince people about OpenID is the fact that your identifier is an URL. Which I find totally cool, but other people seem to think that users don't have enough capacity to understand that they can use some other string instead of just using an e-mail address :? .
Brad Fitzpatrick was clever enough to realize it is possible to use an email address as an OpenID identifier, with some XRDS magic :) .
And now after some months we have Email Address to URL Translation (EAUT).