Well Blizzard Europe is relocating the eight highest populated german realms and the eight highest populated english realms to their own datacenter and will be upgrading the hardware of those realms to reduce lags on those servers and those that were colocated with them.
In order for them being able to do that they have shut down all servers currently located in the same datacenter as those 16 mentioned above and will bring them online again on thursday at around 8 a.m. CET (24 hours downtime).
So they have shut down 41 german realms which is 75% of all german realms and 37 english realms which is 43% of all english speaking realms.
This shutdown has occured without prior notice, players were informed only after the realms were already down via a forum post by the EU community rep and a note on the login screen and the character creation on the remaining realms was not deactivated which means that every realm that is still online is swamped with thousands of level 1 alts that are pissed off and disrupt playing for everybody else. Every server has massive queues and some people even try to purposefully crash servers so that those servers do not get ahead in the race of who opens ahn qiraj first.
All in all chaos ensues and the forum is one giant flamefest.