I'd like to change new account creation and logging in / out to not actually change the page you're on.
Instead, you'll get an overlay with a form prompting you to take whichever action it is you've selected. Completing this will (currently) refresh the current page, and in future just send a so-and-so logged in event to the page causing relevant bits of the page to update.
I'd like to combine this with no longer requiring email verification to create an account, in order to get more participation from casual visitors. (Email verification would still be available and some things (like writing reviews) might still require it.)
This would significantly improve the situation with attempting to post to a forum after having logged your session out from another window... it could just pop up the login overlay when that happens, have you authenticate, and then continue on its merry way.