MMO simply means many people can access the same content at the same time.

WoW is not team sport, such as football or baseball, where you cannot play without a group. It is a solo, yet group-friendly activity, like taking a walk in the park. You can walk by yourself, or you can walk with others. If the other people become tiresome, if they're walking too slowly or quickly, or if they're going in a direction you don't like, you can leave and continue on your merry way.

I have to agree with Cyno a little about the nature of an MMO. Sure MMO doesn't mean "team game only", but I don't really see the point of playing a genre with a sole purpose of putting thousands of people in the same world at the same time if you don't want to interact with them. There are dozens of brilliant RPGs for those who don't like social interaction.

Not to say that I think Blizzard -shouldn't- add solo content, cause sometimes you might want a break from your guild and friends or they might not be online at the time, but it's at least a fair argument that a multiplayer game should focus on content that involves multiple players.