Major difference: Social auth outsources your user table. Persona outsources your password column. You still have your own users, and you still have a portable identifier for them.
What's more, Persona is built as a fully-decentralized architecture with a temporary centralized fallback. That means that one button can support all users, via their email provider's native authentication mechanisms (in the future) or via Mozilla's centralized fallback (for now).
I don't understand your first point. How can you have an app function without a users table?
The main issue as I see it is just the baggage that comes with a private company login. For instance, Facebook login raises questions about what Facebook will do with the login information, and also what the site will try to do with your Facebook data and permissions. Those questions will increasingly become a concern for everyone, but beyond that some people have opted completely out of Facebook because they find it actively harmful and so they are excluded a priori.
Persona is simply a login mechanism with the users interest in mind. People may take or it leave it, but it will be on its own merits rather than being overshadowed by needlessly related general concerns with a third-party product.
What's more, Persona is built as a fully-decentralized architecture with a temporary centralized fallback. That means that one button can support all users, via their email provider's native authentication mechanisms (in the future) or via Mozilla's centralized fallback (for now).