Sever population has always been a touchy issues. Certain servers become end up becoming overpopulated and can feel like your walking down the streets of New York, while other are virtual ghost town.
In the past there where good reason for having multiple servers. You wouldn’t want 100,000 newbie humans, fighting each other in Qeynos hills for a level 2 rat so that they can level up. You also wouldn’t want 20 groups camping Sir Uber Dragon, fighting and training each other to be able to kill said mob.
But over the last 5 years the MMO industry has moved away from static content to more instance content. In Everquest 2 everything can be instances. If 1000 people where to zone in to the Commonlands the game will make new instances of the Commonlands. If a static dungeon has to many people in it, another instance is created. Aion is similar, there are 5 or 6 shards you can move to (at any time). Normally everyone is in shard 1 unless they move them self’s.
Blizzard is adding in their patch 3.3 the ability to group with cross realm LFG tool. other games are allowing you to go in to any open server with your character. (IE your character is no longer pegged on to a single server).
I believe this will be leading to more MMO’s taking the EVE model. IE 1 server for the entire game. With instancing this should not be a problem, except maybe the cities… Since it would really feel like New York. But i believe MMO companies will figure a way around that soon.