Not an exploit per se, more of a glitch in the functionality of the social mechanics (guild, mail, banks). Serious enough for ZOS to shut it down entirely to prevent further damage (because people being accidentally hooked into each others' ingame accounts, with full access to their character and bank inventories, as well as their mail, is bad news bears).
There was no way to hotfix something that broken, so we got the downtime. I'm pretty okay with that - I'd much rather they tackle issues like that head on, and work on a fix, than pretend everything's fine for the sake of the media outlets.
I will be glad to have guildchat back though; Zone chat gives me hives.