We want to streamline the user registration and login process. The goal is to reduce the time and effort for users to register and login to our site.

At the same time, we don't want to overwhelm users with choices. We don't like how some web sites present registration/login options via multiple channels (e.g., Facebook, Twitter).

What are the pros/cons of each of these systems? Which do you use, and what are your main gripes?