Agreed. Although Persona's technical basis and privacy protections are second to none, the UX is nothing to write home about. It still feels too much like OpenID, and we know what happened to OpenID. Facebook and Twitter can get away with cross-site redirects because they're well known and people trust them. Persona doesn't have that benefit, so it can't get away with the same cumbersome UX. It needs to do better, much better. The market is unfair. Deal with it.
If you're in the business of implementing an alternative login system, you should also seriously think about what kind of UX you're competing against. Your ultimate competitor isn't Facebook or Twitter. It's the good old email-and-password login system that everyone is used to. You enter your email address, select a password, and you're in, without ever leaving the signup page! It's even easier if you use a password manager like LastPass. That's what you're competing against, and if your UX has any more steps or redirects than that, you're probably doomed.
The paradox here is that people are more familiar with the appearance of home-grown-style login systems and are more willing to follow through on those than the novel Persona flow, even though the security characteristics of Persona are stronger. It's a chicken and egg problem, and until someone really big takes the plunge and gets everyone comfortable with this style, anyone implementing it is going to be somewhat of a cost to signups.
If the bridge supports the 3-4 major email providers, it effectively becomes "log in with your email address" (it already supports Gmail), and A LOT of the friction goes away.
OpenID never really made it not just because of their bad UX design but also because they never got major players to push it to the public. Google or Facebook would much rather have you use their service as login credentials as it makes more monetary sense to do so than to hand it over to some non-profit foundation like Mozilla or OpenID. Data = money in this world and everyone wants more money.
If you're in the business of implementing an alternative login system, you should also seriously think about what kind of UX you're competing against. Your ultimate competitor isn't Facebook or Twitter. It's the good old email-and-password login system that everyone is used to. You enter your email address, select a password, and you're in, without ever leaving the signup page! It's even easier if you use a password manager like LastPass. That's what you're competing against, and if your UX has any more steps or redirects than that, you're probably doomed.