
Too massive for so few content
The designer will never be able to generate enough content to cope with the population of the game. For example if you are supposed to destroy some computer terminals, but some other people are there and destroyed them first, you are forced sit there until they rematerialized, so you could destroy them again. Bottlenecks.
Procedural scramblers
Procedural generation could help the designer. But the problem with this content is that the designer is limited with a given set of parts that match together. Basically he is just scrambling pieces of terrain, buildings, characters, items, quests, dialogs, etc. After playing a few times the player will find out that the game is always the same, just things are placed in different positions. And the player will focus on finding where did the useful pieces fall, like in a hide and seek game, making the game repetitive. If you go to the extreme of procedural generation, the world will become too weird for the player to understand.
Perpetual tutorial
So to avoid bottlenecks there must be more roads, then the complexity of the world increases. Now, the designer is required to ease the gameplay with guided tours thru the world, but in increasingly complex worlds this is being taken to the extreme of transforming the whole game experience into the longest tutorials. Basically you get a message that tells you to go somewhere and do something. You do this, you get a pat on the back, then you're told to go do something else. Ad infinitum.
Solutions?
Is people actually interested in seen an overcrowded game? Maybe having overly crowded games just make them real in all the wrong ways. Maybe it's the ambition of game designers that actually turned into bad design, something that we are realizing until now that technology is capable of taking us to this extreme. And maybe MMOs should be more single-player. Even if the game is an MMO, you should be able to play alone and affect your own copy of the world. You could still invite your friends to play in your copy of the world to socialize, which to me it's the important feature of an MMO.
Thoughts?