There is some maintenance being done on the authentication server at the moment that may be causing some issues with login. The process is expected to run until 9:30 AM PDT.
For anyone still having issues please restart WoW and see if that resolves it. Otherwise things should be shaking out for everyone and there shouldn't be any more issues logging in.
It's very likely that with a lot of people trying to log in at once that the authentication was hit up relatively hard. That should be eased as people get in.