I was disconnected from the server after quitting a Nexus run on my level 71 Druid on Alterac Mountains. I hit "Leave Party", caught a glimpse of Dalaran, and was immediately disconnected. Now, I cannot log back into any character on the realm without being booted immediately after the game finishes loading.
I haven't yet tried logging into a character on another realm, I'll update the post with the results of that in a bit.
I've already tried the usual disable all addons, delete \Cache, etc., to no avail.
Edit: I should add that the loading bar takes maybe 10 seconds longer sitting at 100% than usual when this happens. In addition, I don't even get to see a glimpse of the game world before being disconnected... it goes immediately from the loading graphic to the login screen. I've had issues with being disconnected upon login since 4.0.1 went live, but it's usually resolved after trying again 2 or 3 times. This incident, not so.
Edit 2: The only alt I have on another server is an 11 Blood Elf Mage, and it disconnects too. So it looks like it's account-wide.