Originally posted by Tachyon:

They're player-hosted servers and 99% of people don't want randoms causing hassles. As such, they play with RL family and friends on password-protected servers.

The password is required, as far as I can tell. If i recall correctly, the "Start" button simply wouldn't light up until after I had typed in a password... is there a way to not have a password? I'd love to just leave my server open to my Steam friends.