We covered some of the achievements and abilities being added before, but the latest update has a bunch of new information in it. Specifically, there are a whole whack of quests being added that relate to the Alliance retaking of Gnomeregan and the Horde retaking of Echo Isles! Also, both Alliance and Horde will be able to do a Twilight's Hammer cult quest line. Click through for an exhaustive list. Also to note: they traced out where the new flight paths for these events will take you. Here's the Alliance map, and here's the Horde map.
It looks like the Alliance are going to get to:
The Horde will get to:
I assume (and am hoping!) that this means we'll continue to have access to the Gnomeregan instance once these quests are done.