We've been getting a steady stream of tips and tweets in about the stability issues plaguing nearly every server tonight, and Blizzard has finally confirmed that something isn't working right.
Most people are experiencing extreme lag both in instances and out in regular zones. There doesn't seem to be any rhyme or reason to it all, just constant server-side lag that impairs things like looting, completing quests, using items, buy items, or trading things to other players.
Pretty much every wonderful thing that could have lag to it does; including the lovely 30 second mount cast.
Hopefully the rolling restarts that will be taking place tomorrow morning at 5:00 a.m. PDT / 8:00 a.m. EDT will fix this. Although I'm hopeful that there would be some quick resets tonight to apply a hotfix or whatever else Blizzard needs to do in-order to make the game more playable.
We'll update this post with any additional information if we get it