Earlier quoted context omitted.
Its the scope of MMO's thats the problem. Building a massive game world, with massive networking requirements is very hard.
http://www.stackless.com/ stackless python See in particular the articles about Eve, who co-developed this to produce a non-sharded world (in other words, one that isn't split into virtualized incidences walling players in different 'dungeons' away from each other). Eve can get away with fudging some of these problems to an extent because it takes place in space and so travel is achieved by use of things like warp ga…
You could do the same thing with a classic MMO by having limited resources in a zone that respawn in real time. It actually a reasonable idea, kill all the gnoll's in that dungeon and they don't respawn. However, it's more fun to for most people to be able to repeat the same dungeon over and over with little effort. You could do the same thing with quick movement between zones but then you get the empty feeling when there are not enough players to fill the world.
It just works better in space, however, awsome random content generation could do the same thing on land.
PS: As to the market's being linked, City of Heroes does the same thing; all shards share the same market.