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…