You bring up a valid point. Based on the description of your problem, I would consider this to be a bug that should be resolved by the SE team by adding a valid SSL cert, either specifically for www.superuser.com, or for *.superuser.com.
Account security is a very important subject, and SE has been investing lots of time and resources into getting SSL ...
Are you using HTTPS Everywhere? If so, temporarily disable it for Stack Exchange sites — it seems to be breaking the login page. I encountered this same error earlier today.
See "Can't login with Stack Exchange OpenID when HTTPS-Everywhere plugin is active - apparent protocol mismatch" on Meta Stack Overflow