The implementation at browserid.org returns the email only. So there's no way to change the email without visiting all the sites you've registered in using browserid.
Do an online poll and ask people "Do you know your openID" vs "Do you know your email". Users are trained to use emails for login, and that's a significant investment already.
OTOH, facebook id != identity, neither is twitter username. There will always be some id that you won't be able to change. But you are right that email is a bad choice, and i was surprised they give away the email address to developers. They should provide the browserid.org ID only.
it's a compromise move-- the reason most sites today rely on email, fb, or twitter is that it's a way to 1) contact the user, and 2) tell/help the user to tell his/her friends about something.
the team is in talks now about cooperation with another mozilla labs project centered around 1-off email generation for quick and easy anonymity with sites
They give you a token, you do a GET request to browserid.org with it and get the user's email, that's all. The user has to register their email with browserid.org first.
The third-party service handles the case where the user's browser doesn't handle BrowserID natively, which makes it possible to adopt this service without waiting for user's browsers to catch up.
The implementation using browserid.org, to an end user, will not get any easier or harder than OAuth or any other third-party service. On the other hand, a browser-based implementation could simply bring up a browser notification saying "Authenticate to example.org using your email address?", to which you could click "yes".