When I visit a SE site via plain http (such as http://meta.photo.stackexchange.com), I appear as logged in, so it means that the cookies were not set to secure (when I used other SE sites through https) and are transmitted in plaintext in case any such link is followed.
Combined with the fact there's no way to force session expiration (when logging in with Google), this is a major problem that needs to be fixed.
At the very least, cookies must be set to be secure.