If you played The Old Republic, you’d remember how dead the planets always felt due to BioWare instancing every zone in the game. You could run around questing and almost never see another human player doing the same. It made the game feel more like a single player game than a MMO, even though you were playing $15 a month to access it. That feeling is one of the reasons my entire guild quite TOR.
World of Warcraft never instanced outdoor zones, only dungeons, as Blizzard was smart enough to realize what made a good MMO and part of that was interacting with other players. However as players level past lower-leveled areas, those zones could become pretty sparse on some servers. So with the coming of the new Mists of Pandaria expansion, Blizzard has found a solution of underpopulated zones to make leveling up a more social and fun experience for both new players and those level 85s who will be rolling Panda alts with the expansion.
Starting very soon in the beta, Blizzard will be enabling cross-realm zones:
In the World of Warcraft: Mists of Pandaria beta, we’re introducing a new technology that will enable players to meet up and group with other players in under populated areas of the world such as low level zones or areas that other players may have outleveled or moved on from. This technology will allow players to form a group with other players from within a select pool of realms in order to quest just like they normally would while still allowing the social structures of their home realms to remain intact.
So in one way, servers really don’t mean anything now. Especially if this sort of thing works out.