Frankly, it's not just the big bad syndrome at work, either. That's certainly a part of it: We're conditioned by The Burning Crusade, Wrath and Cata to think of expansions like series or seasons of a television program, with an overarching threat at the end that ties the experience up with a nice little bow and unifies what we did. While The Burning Crusade effectively subverted this itself with the Sunwell Plateau raid (dethroning Illidan as the end boss of his own expansion), both Wrath and Cataclysm held true to the paradigm. Really, they almost had to. Even though there was a raid after ICC, no one mistook Halion for anything but a teaser, and there's nothing going on raid-wise in Cataclysm after Dragon Soul.
With Mists of Pandaria, we're returning to the feeling, if not the execution, of classic World of Warcraft, with the world itself and our exploration of it being the focus. This doesn't mean that the previous expansions didn't have plenty of world to explore. But Mists of Pandaria sets up the theme of players and their actions and what they do to the world as well as what the world does to them. The world is full of wonders and terrors, and rather than picking one and building the expansion around it, we get to see what trouble we can get into.
I find this a fascinating return to form for a few reasons.