We considered many different authentications and chose Facebook and Twitter because they make it really easy to show avatars of the people who sign.
What alternatives would you suggest?
A simple registration form with some default avatars would be ideal, in my opinion.
Another factor was the really short time we had for development, so implementing those two allow us to cover the largest part of our target audience with a simple UI.
Email address + gravatar.
How about just asking for a name and email like everything else?