Failures of storytelling:
Not every story in an MMO is told well -- in fact some are down-right pathetic. The biggest mistake game designers make is substituting in reams of text where immersive gameplay should be. What we actually get in MMOs today is very different to the rosy scene I described earlier. Instead we'd get a lengthy story about the enemy invasion that takes ten clicks to get through, followed by a quest to kill the invaders. Where are the invaders? Are they about to land on shore in boats via some scripted event? No, they're at what looks like a temporary campsite about five feet away from the shoreline. They're trapped in limbo, condemned to stand perfectly still in a perpetual state of "just about to start the invasion". Or worse, they're standing on the shore next to defending soldiers, repeating an attack animation twenty four hours a day.
In both versions of the event, the storyline is the same but the way we're involved in the story is completely different. In this version of the zone, the story is just flavour text used to dress up a few kill quests. In that respect, the zone won't really have its own feel and the story will never come alive. The player isn't immersed in the story, he's just reading it piece by piece while he kills monsters to level up. This is what we get in so many MMOs because developers have failed to adequately consider the role they're trying to put the player in. If the player's meant to be fighting off invaders, they should feel like that's exactly what they're doing. The player shouldn't feel like this quest or zone is the exact same as every other only dressed up with some new flavour text.
What's in a game?
A large part of the fault rests with game designers. In their job, the daily grind involves designing gameplay systems and technical game mechanics. As an amateur game designer myself, I know how easy it is to get bogged down in clever mechanics and forget that they're only there to support a story. The mechanics themselves aren't the game, they're there to support the game and the game is about playing a role. Whether you're playing the role of a wizard in a dungeon group, a spaceship pilot evading pirates or a superhero fighting organised crime, the game is all about playing that role and having fun. The game mechanics are essentially only there to arbitrate the outcome of in-game events. They decide what happens in a simulationist manner, quantifying your actions and calculating their consequences.
Problems arise when game mechanics take priority over the story roles players are placed into. Game designers aren't just responsible for creating balanced and interesting game mechanics that work. Their job is to carefully craft a game experience based on how they want the player to feel -- to create a role for the player to fill. This counts as much for class and race design as it does for quests and zones. If you want the player to feel like a wizard, it'll take more than fire damage, funny-looking robes and a pointy hat. The game mechanics need to make him feel like a wizard and so place him adequately in that role. When I think of wizards, I think of chanting spells in unknown tongues, spellbooks with runic lettering, powerful magic staves, pentagrams drawn on the ground and teleportation. I sure as hell don't think of pressing 1 to 4 repeatedly and then picking up the loot. It's up to the game designers to create unique mechanics for every class matching their role and practically every MMO falls far short of the mark in that department.
The story in MMOs is too often limited to pages of text in prelude to a quest and a short epilogue afterward. I can't speak for everyone but I know I don't play MMOs to perform mundane tasks or solely to play with interesting game mechanics. I play them to get away from reality and take part in fantastic events and stories alongside a few friends. I don't play because I like to kill orcs, I play because the orcs are advancing and we absolutely have to stop them. The roles game designers put us in are often so far from this ideal that it's a major disappointment. These aren't the roles we imagine as we play pretend, nor are they the roles developers fill trailers and lore with. These are the roles we actually play, and they're not good enough.