After reading some recent posts from Galoheart and Big Bear Butt Blogger, I have solidified my view that the central paradox of MMOs is:
The best thing about MMOs is that you can play with other people.
The worst thing about MMOs is that you have to play with other people.
In many ways, WoW was the first MMO to really grasp this distinction. And a huge amount of the angst and conflict at the level cap comes when the game transitions from focusing on the first style to focusing on the second style. We play this game to play with other people, yet playing with other people often causes a lot of problems.
It's amazing fun to play with other people. It's what sets this genre apart from all the other games out there. I still believe that single best thrill in WoW is downing a hard new boss with a raid of friends. A hard-fought, evenly matched Arena battle might compare. They're both the same idea: defeat a hard challenge because you worked together as a team.
Yet other people are also the worst part of MMOs. Drama, people going afk, griefing, etc. So many problems in this game aren't really problems with the game itself, but with people. And sometimes it's just simple logistics. Person A only has 20 minutes to play, so that's just not enough time to do something as a group.
There are a ton of issues where the difference comes into play. Take raiders vs casuals. Raiding demands that you play with a large number of other people, and thus places strict demands on people in order to make the playstyle reliable. Casual places fewer demands, but you often end up playing by yourself.
PvE vs PvP. The real advantage PvP has over raiding is that it involves fewer "other people". Battlegrounds you can essentially solo. Arenas involve 1 to 4 other people.
The shortage of tanks and healers. DPS "can" play with other people. Tanks and healers "have to" play with other people.
Realistically, what can a game company do? The entire point of these games is to play with other people, yet the more you force people to play together the harder it becomes to consistently enjoy the experience. I know there are other MMOs who force you to group all the time, but I also believe that is a reason they haven't achieved the success of WoW.
In the end, all I have to offer are the (paraphrased) words of the great philosopher Homer J. Simpson, "other people: the cause of, and solution to, all of life's problems."