1) Forced all users in your system, who already had email/password based accounts, to abandon their existing accounts and adopt Persona
2) Became fluent in quickly explaining to users how to reset their Persona password when they forget it
3) Don't mind losing all the users who say "I trust you, but not this company I haven't heard of before." Some users, especially jobseekers, won't let their circle of trust grow - and forcing adoption of a 3rd party causes abandonment. But so does a complicated sign-in process, so you just need to figure out if group A or group B is larger because you can't make everyone happy.
#1 makes no sense. If you already have email-based accounts, switching to Persona is trivial - you just switch to Persona auth and everything works as-is with email as the account key. You will of course want to give users a little warning, but the process is far simpler than almost any other change you can make to your authentication system (eg, deconfliciting usernames). The only real change when switching to Persona is that you drop your stored password. It's still "sign in with your email address".
#2 has not been a problem, as far as we can tell. If users forget their passwords, the reset-password process on the Persona login dialog works great - better than almost every other password-based website I've used. In the long run, passwords are only maintained by primary IdPs like Gmail, so this is even less of an issue.
#3 is totally dependent upon your audience. We definitely get pushback on Facebook auth - you can't run a FB-only login system without alienating a significant chunk of most audiences. Initially we tried to encourage FB auth by making BrowserID less obvious, but that just produced a lot of angry emails from people who didn't realize they had an alternative. Now that Persona and FB auth are on equal footings, we have yet to have anyone complain about the signup process. YMMV.
1) Forced all users in your system, who already had email/password based accounts, to abandon their existing accounts and adopt Persona
2) Became fluent in quickly explaining to users how to reset their Persona password when they forget it
3) Don't mind losing all the users who say "I trust you, but not this company I haven't heard of before." Some users, especially jobseekers, won't let their circle of trust grow - and forcing adoption of a 3rd party causes abandonment. But so does a complicated sign-in process, so you just need to figure out if group A or group B is larger because you can't make everyone happy.