I am a big fan of the idea of having one world for MMOs, and I don’t mind if they use instances to achieve it. The biggest concern when it comes to breaking the world up that way is the potential loss of community. If all 100,000 of your users are on the same world, and they all go to town at once, your game might have 100 instances of that town (as opposed to needing 100 servers to make sure your population levels are such that town doesn’t exceed 1,000 players at a time), the worry is that the 1,000 people you are in the instance with will likely never be the same 1,000 the next time this happens.  Even if only enough people ever go to town that never more than 5 instances are needed, the chance you run into the same people over and over is pretty low.
Fact is, even on a game that limits players to 10,000 per server, no one knows everyone. But finding your 100 “friends” out of 10,000 is easier than 100,000 or 1,000,000, even easier when only a portion of that 10k plays in your timezone. An instanced game gets even worse if you leave town and enter an instance designed for 75 people max, the chances you’ll play with the same 75 people is even smaller. So how do you meet new people and make friends? How does a community build when everyone shares?
It would be nice if someone could take the one world/instance design and then pair it up with a player matching algorithm, so that if you play with someone in a group or raid for any signifigant length of time they’d earn a rank, and you can flag players as good or bad, coupled with your friends list resulting in the game choosing an instance with the highest matching score. Â This way, you would tend to play with people you’d played with before, except of course when the game cannot let you (instances exist for a reason, and sometime you just can’t let more people in), but you can allow for player overrides so even if the game chooses to put you in Wilderness Instance 27, you can swap to join a player you know who is in Wilderness Instance 19, or they you.
Its a thought… just need to figure out how to build it…
In a properly designed world with enough places to act as market hubs, then populations would learn to avoid the all-going-to-one-town problem.
Take Eve for example: its main market Jita gets a lot of people going there but secondary markets have grown and propspered as sellers and buyers wish to avoid lag at the main hub. A balance is reached where the right number for the hub’s server is almost always there but no more. In Eve, that means about 1000 people in Jita.
That works very well in a game like EVE where the world itself is almost irrelevant. Aside from different systems having different resources and stations, they are all generically the same. But in a Fantasy setting, as opposed to a Sci-Fi setting, people expect towns to be more, to have personality, to be more hand crafted. In World of Warcraft, after they added the auction house in every city and shared them, people tended to go to the city they were more stylistically drawn to. But Stormwind never became as popular as Ironforge, due in part I would guess to Ironforge be so much more dramatic in its style, where Stormwind is more subdued. If they were to add a dozen more Ironforges, not as instances but as new areas, the beauty of Ironforge would be less, and of course it would impact the lore. EVE gets away with it because there is so little place based lore, the majority of their lore is race based and occurs in a distant past.