The concept of mirrored online worlds has always bothered me. It segregates the population into neat, little packages that are all basically exactly alike. All MMOPWs (Massive Multiplayer Online Persistent Worlds) do this, with a few exceptions. I well understand the technical limitations that has forced this state upon us, but that doesn't mean that a solution can't be found with a little creativity.
And so we come to Shadowbane. Wolfpack had an incredibly clever idea regarding this problem, but, unfortunately, they never followed through with it. The backstory for the game was that the world had been broken into pieces, and that these pieces floated through the void but might be connected via special gates. Each 'shard' was to have a different map and possibly a different ruleset (one might give bonuses to fire magic, for example). This is a brilliant solution in my opinion! You still have segregated server clusters to distribute the bandwidth and processor load. But each server is markedly different, and there is a possibility to travel between them. Each map can have it's own history and flavour (and Shadowbane had enough interesting backstory to really make this work).
Due to the time required to create multiple worlds, it does seem that this concept would lend itself better to the games that are relatively sparse and built to facilitate player-generated content rather than the amusement-park worlds such as World of Warcraft.
But I really think that this is one of the best ideas that has never made it into a game. Hopefully, someone somewhere down the line will pick up the torch and run with it!