MMOs resemble work too much. And they resemble real life too much.
The Grind. The producers can't possibly create 20-60 hours of good, fresh content a month with a $15 USD subscription fee, so they space out the content by making players slowly perform mind-numbingly repetitive tasks to get it. One endlessly battles the same enemies or harvests the same resources in hopes of finding something marginally worth keeping.
The Roles Not Played. I play video games in part to play the role of the hero or villain, to become for those hours the person who saves or destroys the village, the city, the country, his people, the world, the galaxy, the universe, or even the multiverse. To become someone that matters, someone who makes a difference, as opposed to real life, where most of us have the mundane fates of regular janes and joes and jobs that don't involve saving the world.
The structure of an MMO makes this impossible. Every player can't play the hero. Only one person or party can save the world; not everyone can become the best. As Syndrome said in The Incredibles, "[...]because when everyone is super, no one will be." I suppose you could instance the whole game world. And then you'd just have a single-player RPG online. With more grinding.
The Stories Not Told. This problem outweighs the others. The aforementioned grind/content issue limits story variety and depth. The inherent structure of an MMO (real-time play, PvP, and grouping) kill videogame storytelling conventions and make it difficult to tell a party-based story. What if a member crucial to a major plot point can't play when the other members of the party do? Ways may exist to counter these factors, but to my knowledge nobody today does it.
Apologists will reply that the player makes the story. They'll point to events like some of the great swindles in EVE Online or a particularly brutal crushing of an enemy in Ultima Online. Wrong.
The sort of events described have problems. First they occur only rarely, second they require unreasonable effort, third they have a dark side, and fourth they reek of banality.
These events merit attention because they occur so rarely. All but a tiny fraction of players will never experience events like these. Most players will simply grind their play time away for as long as they stay. I meant thiswhen I said MMOs too much resemble real life.
Then we come to the effort required. In order to have even a chance at that kind of epic win, the player will have to spend untold hours grinding to get a character with the necessary stats. I meant this, and the grind in general, when I said MMOs too much resemble work.
These events have a dark side: every epic win comes with an epic fail. Every gank has someone who got ganked. Every swindle comes with a number of people who got swindled. Every brutal crushing has a crushee. And in some games, merely recovering from such a blow can take months. Months that feel like work, not play. So for the majority of players that participate in these events, the story has an unhappy plot with a bad ending. Again, it too closely resembles real life. And work.
And finally we see the banality of it all. Most of these events have no plot, no theme, no interesting characters. They have just the gankers, the gankee, and the gank. The swindle comes closest of any of these to the requirements of a story, and only the swindler gets a good one.
The Battles Not Fought. In part, people play single-player RPGs - and videogames in general - for the fun of combat. Much of the fun in a single-player RPG lies in picking the right combination of party members, attacks, weapons, and spells to defeat the enemy party. Most MMOs have an auto-combat system that involves picking those things in advance, then when combat begins waiting for your character or the enemy's to win. It sucks all the fun out of fighting.
Abandon All Hope? Not yet. All of the problems I've outlined above may have solutions or work-arounds. Game producers can reduce grinding by making additional content unnecessary, by implementing procedurally generated content, by giving diminishing returns for levels, by introducing handicapping systems, and so on. MMOs can compensate for the lack of messianic roles by making more ordinary roles fun to play in other ways, perhaps by giving them cool and unique items and combat moves. They can fix the story problems by eliminating the need for stories, or make stories part of the environment without giving the player any crucial role. Lastly, no reason exists why MMOs cannot have interesting combat, whether turn-based or real-time, that more directly engages the player.
I have heard of some MMOs that solve one or more of these problems, and intend to look into them further. Guild Wars and Age of Conan are supposed to have interesting combat. Puzzle Pirates (which I mean to try) attempts to solve all the issues by making puzzle-solving the central activity, thus greatly reducing the impact on the player of grinding, roles, and story.
I welcome comments, especially those naming MMOs that have solved, reduced or avoided these problems.