In WSN it's always possible for a member to login with their email in place of their username -- username and email are accepted equally. Still, sometimes you may not want to have a username field at all and prefer to have people login just using their email address and password. Google, for example, does logins based on your email. To do this with WSN, go to Admin Panel -> Settings -> Switches and turn on 'treat emails as usernames'. Once on, your username won't work to login anymore -- only your email.

Note that this switch works nicely in conjunction with the "autogenerate accounts" switch to provide people with the ability to edit their topics without their having to do anything but include their email address in their topic submission.