One of the big benefits to playing World of Warcraft has always been that it can be played on a variety of systems, from the exceptionally small and outdated to the current, top-of-the-line models. I always found that a fascinating phenomenon, and it made good sense -- after all, if you want the max number of players able to play your game, you want to make it available with the widest software and technology possible. Yet there's a drawback to this; if you want to continually make that content available for older systems, there's only so far you can go updating content.
As a game that just celebrated its seven-year anniversary, WoW absolutely needs to keep updating in order to remain competitive. With new MMOs coming out all the time, a game that is seven years old starts to lose its shine. Continuous updates make sure that it stays just as fresh as it did in 2004. You can't keep those updates rolling if you're trying to support an operating system that is now 12 years old. I think, however, we're safely in the territory where most people have moved beyond Windows 2000, so this shouldn't affect a huge majority of players, particularly when Microsoft itself has already phased out support for the operating system. What this does do, however, is make me look forward to Mists of Pandaria and what it has in store.