. Religion is returning in a meatier form and so is espionage, there's a new diplomatic system with city states being more important, new civs, units, wonders, and all the other usual stuff.
Just yesterday I went checking the Civ Fanatic forums to see if there were any rumours on an expansion and when I didn't see anything I pretty much gave up hope on ever seeing one. They sure timed this announcement right.
Are you a member of CFC? I'm smellincoffee there, same as here. I'm still playing Civ3 (and sometimes, CIV), but mostly hang around in off-topic.