It would just block the site to everyone unless they have signed up. You would still have your usernames and such. Nothing would change except that the lurkers couldn't lurk.. Anyway, it seems that most are against the idea as I have received a few personal messages also. This being here for new people to find seems to be the biggest concern.. If it were private, searching would not find it..
I just saw a couple of threads that could be detrimental to the poster if anyone in their office happened upon here to read it. Just wanted to gauge what the community wanted to do and it seems to keep it open is the winner..