GameSetWatch pointed to a nice little blog post about the trend in MMO gaming to make the player feel "special" -- depsite the fact that there is nothing special at all about any given player in most MMOs. I'm not sure where I fall on this issue, but my instinct tells me that the player must be given some reason to feel important in the game...otherwise, why play?
There is a suspension of disbelief that one must have when dealing with current MMO design. For example: I did that quest and saved the world, you also did that quest and saved the world. It's an inconsistency that most players are willing to tolerate. But the next great MMO design will feature elements in which a single player can truly feel "special" and have a significant impact on the world.