Personally, I think one of the most exciting undocumented changes in 2.4.3 is the work being done in Stormwind. The action in the Alliance's heartland isn't limited to the comic book, oh no. Alliance construction workers are breaking their backs day and night to make preparations for the upcoming addition to Stormwind City, the Stormwind Harbor. You can take a look on the PTR between the Park and the Cathedral. I can't help but notice the boatloads of explosives floating in the canals. You would think they would take the back wall of the city down the traditional way, but it looks like the Dwarven workers may be taking lessons from the Gnomes.
Hopefully, this is only the first in a long line of changes to the world leading up to Wrath of the Lich King. Something like that definitely feels like the progress needed in-game to lead up to such a meaningful expansion as far as story goes. Such massive changes to one of the most beloved Alliance cities shouldn't simply happen overnight. We'll see if they continue this while we're on the road to Wrath. Who knows, we may even get a few hints at other world changes we aren't sure about quite yet.
Gallery: Patch 2.4.3