Due to their slow production cycle, World of Warcraft expansions very much suffer from a phenomenon of players unsubscribing after running out of things to do, and resubscribing when the next expansion adds content. Cataclysm having come out this week, resubscriptions are coming in fast and heavy, and servers are beginning to feel the load. During prime time their are queues on many servers.
Unfortunately that load isn't even. People who resubscribe usually want to continue playing on the server where their characters are parked. Thus for any given server the number of people coming back is roughly proportional to the number of people that unsubscribed previously, and that is roughly proportional to the age of the server. As the server I'm playing on is one of the original release servers, the volatility of player numbers on my server is higher than on one of the latest servers. Not only is the server full now, but it also tends to drop in activity between expansions more than younger servers do.
Except for games playing in space, it appears that the technology to have all players on the same server in a MMORPG doesn't exist yet. Many problems of server population have been solved by server clusters, which at least allow players from different servers to fight each other in PvP, or together in PvE instance groups. But that still leaves important barriers between servers. Changing servers costs money, takes time, and cuts social links, as you can't chat or guild with people on other servers.
While obviously it won't be possible for Blizzard to completely change their architecture to switch World of Warcraft to a single-server system, I still wonder if there aren't some improvements possible. Why can't server changes be automated to a degree where Blizzard can offer them for free, and happening within minutes? And why can't we have chat and guilds that span at least a server cluster? If changing servers were easier, population would automatically even out between the servers of one cluster, and there would be less problems of queues and population volatility.