Personally I think the biggest issue will be determining how and which external login providers we support. We should establish criteria that all external login providers must meet, and then a process for adding new ones (are we going to require a minimum number of users before adding a service?, etc.). And how we display them on the login/create account pages themselves..do we want to make the most popular ones more prominent (Google, Facebook, etc.) or can we be neutral and just alphabetize them or something..
As a start, I think we should only be supporting login providers that use a standardized auth mechanisms (OAuth, OpenID, etc.), no proprietary protocols.
And RIP Persona :(