In single-player RPGs, the designer has an advantage in being able to have the world revolve around the player. Everything the player does can be reflected in the world, because the player is the prime mover. And when the player gets to the end, the storyline is over. That model doesn't work in an MMOG for a number of reasons, not the least of which is that there are thousands of players. It's hard to feel special and unique when there are thousands of others doing the same thing, Cain said.
On top of that, there are issues with stories that span multiple zones. Zones can be skipped in some MMOGs, and players can overgrind an area of a zone and become too powerful for the rest of the zone. There are plenty of failed attempts to tell MMOG stories. Designers tried cramming the story into "lore bombs" but found players would click through the written text without reading it.
Having instances makes the world seem disjointed, Cain said, and it undermines the shared-world experience of seeing so many other players around the gameworld. MMOG players also dislike the action being ground to a halt for the telling of a story, and they're not always interested in a story that ends. Finally the sandbox approach that works well in games like Fallout is tough to do in an MMOG because emergent stories based on thousands of players are incoherent.
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