* federated (like OpenID)
* open standard (like OpenID)
* no passwords / no typing / no memorizing (e.g. like FB Connect)
* possibility of browsers providing an integrated experience (technically possible with previous solutions, but no browser has done this so far -- in the case of OpenID for a very good reason IMHO)
* anonymity/choice of identities (like OpenID, definitely unlike e.g. FB Connect)
* no exposure to identity provider (this is unlike any existing solutions; if you log into a site, your OpenID provide, Facebook, Google, etc. will know which site it was; not with BrowserID!)
Check out Dan and Ben giving a nice demo of the BrowserID user experience: http://www.youtube.com/watch?v=6x45Nt1fOMM. No passwords, no typing, and anonymity where desired.
If you're interested in the nitty gritty details, Lloyd explains the cryptographic assertions that actually let sites verify your identity: http://lloyd.io/how-browserid-works
(EDIT: format bullet points)