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.
First off, we're not locked into a final confrontation. Not that we won't have one, but there's something to be said for not knowing what it is. Naxxramas-40 was for all intents and purposes the final battle for classic WoW, and Kel'Thuzad was the level 60 game's end boss. But we only found that out when the Shadow of the Necropolis patch hit -- and even then, for all we knew, there could have been several more raids.
The unfolding of dungeons and raids in classic WoW happened in patches that were clearly part of the same world (for instance, in order to complete the legendary staff Atiesh introduced with Naxxramas, you had to kill C'thun in AQ-40 and then go to Stratholme, in addition to collecting fragments in Naxx itself) but that were not intended to thematically reflect one another. We went to BWL after MC because it was there and there was a great big dragon in it. We didn't need a reason aside from that.
It's not as if the game did away with this kind of thing; it just tended to focus on it less. For instance, in The Burning Crusade, neither Karazhan nor Zul'Aman had anything to do with the overarching Illidan/Burning Legion plot line that was behind our voyage to Outland. In part, that's because neither are in Outland. Oh, sure, there's Prince Malchezaar at the end of Kara -- but be honest, no one has any idea what Malchezaar was doing in Karazhan besides tromping around upstairs and failing to drop my axe. And despite personally killing him, I still regret that Zul'jin is dead, but you can't argue that he had anything to do with Illidan or the Legion at all.
To go where you haven't been before
Similarly, while Wrath of the Lich King absolutely had a strong theme and aimed squarely at Arthas, there were quite a few digressions like Obsidian Sanctum, the Eye of Eternity and Ulduar that pursued other stories. Cataclysm stands out as being the only expansion where all of the raids and most if not all of the dungeons were related to the end boss. (OK, so The Deadmines is more justified by the Cataclysm than it is directly related to it.) Bastion of Twilight, Blackwing Descent, and the Firelands are all related in some way to Deathwing, be they through his Twilight's Hammer minions, his offspring, or Deathwing's action in summoning Ragnaros to Hyjal in the first place. Even Throne of the Four Winds sends us in to destroy Al'Akir because he is Deathwing's ally.
There was really no Karazhan or Ulduar in Cataclysm, and I think the game suffered slightly for it. As bad as the Lich King was, the entire world didn't revolve around him, and life continued with its mysteries and its dangers. I think that taking a break from having a singular menace looming over the world in Mists of Pandaria is a solid thematic choice, and it will allow each raid tier to be its own experience, each dungeon to be its own adventure.
Frankly, I'm looking forward to going somewhere new and seeing what there is to see and do there without the looming presence of Archlord Evillaughter always reminding me who I'm going to be killing at the end of everything. This way, it's a surprise. It doesn't mean there's no story or that there's no lore to be had. It just means you don't know how it's going to end until you get there. I don't see any problem with that.
It's open warfare between Alliance and Horde in Mists of Pandaria, World of Warcraft's next expansion. Jump into five new levels with new talents and class mechanics, try the new monk class, and create a pandaren character to ally with either Horde or Alliance. Look for expansion basics in our Mists FAQ, or dig into our spring press event coverage for more details!