If WoW -or any other current AA rank MMO- was a single player game, I wouldn't most probably play it. At least not the gear min-maxing game, which is the most apparent one in the level cap.
In fact, I don't know a single single player game which would require the player such a dedication, information digging and preliminary preparation as playing at level cap requires at the moment. In a sense the 'fun' is lost for the most part due to the fact that every time I log in I'm faced with the obligatory requirement to have learned the fights off game, tweaked my gear beforehand and/or created a 'wish list' for the min-maxed gear setting.
If I do not do this, I will be shunned upon, my gear and performance will be evaluated before entering the dungeon, instance or group and even if deemed proper I will be held responsible for the poor performance of the whole team due to the fact that it's always either tank or healer who blunders.
The more I think of it, the more apparent it is that there is something else in the game that is keeping me logging in than the co-operative game (which feels more abuse than pleasure): I haven't yet grasped what it is in tangible terms, but when I find that out, I'll tell you. But it isn't the group playing except with guildies, as in parties the players are most of the time for themselves: either to top the damage meters, to ninja the next piece of gear or just pass time and whine about others. Especially newbies who are running the dungeon for the first time are pointed at as being "n00bs" and usually claimed to be the fault when something goes wrong.
I mean, how many times have you stated that someone was either a noob or just a loser in a group, causing a wipe after a wipe, and forgot to see your own part in the whole? How many times have you started the evaluation from the point of view that there might be something you could do otherwise or -God forbid!- wrong?
There must be a decent amount of masochist in me for letting this go on and making me log in voluntarily to the cesspool of disappointment time and again. But then again, there are millions of us who are chasing the digital holy grail day in, day out, just by logging in and moving some bits across the screen.
Still, if WoW was a single player game with the population and itemization it currently has, I wouldn't be playing it.
I would still be playing Oblivion.