This is a video released by the World of Warcraft that covers the main features of patch 5.4: The Timeless Isle, Siege of Orgrimmar, the Proving Grounds, and more. Check out Blizzard’s page for the patch notes which detail most of the changes. (I’m sure there are a few undocumented changes, though they’re probably small.)
Enjoy:
And then you should make sure you’re leveled, in order to get at all this content.