It's common knowledge now that OpenID has severe usability problems. As much as I wanted it to work, none of my apps' visitors (hell, back in 2006) could understand what it was or how to use it. BrowserID, however, has now fixed most of these issues, and it's very straightforward to use. To try it out, you can have a look at http://www.yourpane.com/
"To log in, just enter your email address below and click the link that we send you. To log in again, you can save the link and use it directly (you don't have to request a new one)." In what world is that more usable than "Click here to login with Google / Yahoo"?
If you want to see a more "pure" Persona implementation, our standard example site is at http://123done.org/ (and you can preview the "Persona" redesign at http://dev.123done.org/).
As for as being as easy for first-time users as "Click here to log in with Google / Yahoo," well, we've got some tricks up our sleeves. Look for a blog post next month at http://identity.mozilla.com/ :)