Thematic diversity of areas. No two areas are the same. Stylistic elements are TO TEH MAX!!!1!!1!!eleven!!!.
At first, it was a little jarring. Everything was rich, extreme, unrealistic to the point of fantastical. But then again, that's part of the draw. It's a tacit rejection of realism in favor of artistic fantasy. Aesthetics, not consistency, is the important thing.
Each area's flavor effects its narrative description and backstory.
Undead areas are dark, mysterious, and melancholic. The world mythos for these areas are rife with pain, grief, and tragedy. In MMORPG's you often have quests that reflect a glimmer of hope that things can be different. A vengeful spirit that can only be put to rest with a love letter from it's ancient lover; a font of evil that can be destroyed, etc.
In an MMORPG, you can have each area have its own ancient mythos that the quests eventually tie into. But then there's the meta-quests that tie them together. Each continent has two or three of them. They form a chain of quests that take adventurers from one area to the next. They can fulfill the quests in order, or they can stumble into a middle link and follow it from there. OR, they might stumble into another meta-quest. Eventually, though, the meta-quests tie together, so that no matter what road the adventurers take, they end up at the world-shattering event that everything thus far has led up to. In MMORPG's this is called "end-game content," and is structured in such a way that it is only partly resolved, so that the story-line can continue with the next software expansion.
In my opinion, those games that can learn the most from MMORPG's are sandbaox games where events are happening in the world that adventurers need to interact with. The storylines are laid down already, and it's up to the players to decided where, when, and how they are participating in world events.
It's something I've been considering for my next campaign.