(This is essentially a rebuttal idea to this feature request because as stated, it's not possible to implement.)
What I'd like is the ability to manually control which sites I'm automatically logged in to.
I like to keep my accounts logged out as much as possible. Often I'll open a link to another site for reference, which automatically logs me in to that site. Before I close the tab, I have to remember to manually log out every time. It would be simpler if I was never logged in to start with.
As auto-login is a nice feature for lay-users, I'm not going to suggest removing it entirely (not that that will fly anyway), but for power users like myself, I'd like that extra bit of control over my credentials.
I suggest this feature be implemented as a simple checkbox in the user preferences (which would make it work on a per-site basis). Having it defaulted to enable auto-login, there would be no end-user change from the current state of the system.