Presumably your value computation includes values other than "enjoying doing something". For example given the explicit choice between getting to enjoy delicious food for the next 5 years at the expense of having a child starve, versus having average food and letting the child live, most people would choose the latter - even if they were extremely confident that there were no ulterior consequences of any kind and the food would contain additives to surgically neuter any guilt they might otherwise feel.
The case regarding your playing WoW instead of doing something else may be less clear cut than that, but one example is enough to prove that there is nothing unusual about valuing something more strongly than one's own enjoyment of some activity. A related Less Wrong trope is "orgasmium", which is considered by most to be an undesirable future for humanity despite its being defined as a state of optimal bliss.
Whether or not playing WoW is actually what you should do depends on your what your other values are, besides wanting to enjoy WoW - and you probably know more than anyone else here about those values; although since humans generally possess similar values, depending on your ability to introspect and imagine your response to certain arguments and stimuli you might not. After all, shouldness is a two-place function that takes as one of its arguments your own brain. It also depends on how accurate your model of reality apart from your value computation is (for example, if you had never taken drugs you might be unaware of how pleasurable, or displeasurable, that is).
But as far as your values are similar to the majority of humans, WoW loses out to other, stronger values a lot of the time because:
a) it is inconsequential and unproductive
b) it is seen as a solitary activity
c) it is aesthetically displeasing in lots of other, small ways, e.g. the physical position of the human, his lack of attention to the environment, the narrow range of emotion, the pandering to the lowest common denominator/ mass consumption, the dependency on interacting with someone else's software cynically created to maximise addiction instead of beauty or inventiveness...
d) it is time-consuming, compared to many other hedonistic activities
Also consider the distinction between wanting and liking - although most people are not explicitly aware of this distinction, they probably have some intuitive feeling that they do not "enjoy" WoW so much as they cannot help but do it.
For example given the explicit choice between getting to enjoy delicious food for the next 5 years at the expense of having a child starve, versus having average food and letting the child live, most people would choose the latter - even if they were extremely confident that there were no ulterior consequences of any kind and the food would contain additives to surgically neuter any guilt they might otherwise feel.
Er, isn't that the choice most people face, and don't most people choose tasty food? Even people with ascetic diets- like myself- seem to do it primarily out of the joy they find in asceticism rather than to free up productive power than others.
I've avoided playing World of Warcraft because many people enjoy it so much that they neglect other things in their life.
Does that make sense?
How about cocaine?
How about sex? I hear that's pretty good too.
ADDED: Lots of interesting discussion, but no one is getting at some points of particular interest to me. Most answers assume that you have important stuff to do, and you need to decide whether WoW will prevent you from getting that important stuff done. They also assume that your brain usually errs on the side of telling you to do "non-important" stuff (WoW) at the expense of "important stuff".
One question is whether there is any evidence that your brain is biased in this way. I think your reflective self greatly overestimates the probability of success at the "important stuff". I have worked very hard, twelve hours a day, 7 days a week, on "important stuff" for most of the past 30 years. The important stuff never pans out. So it appears that when my brain told me to play Freecell rather than work on that important paper on artificial intelligence that got pulled from the book the day before publication due to petty office politics, or to watch Buffy rather than do another test run of the demo I spent three months preparing for DARPA that no one from DARPA ever watched because the program officer was too busy to supervise his program, or to go hiking instead of spending another weekend working on the project for NASA that was eventually so big and successful that my boss took it over and then tried to get me fired1, or to go dancing rather than work on the natural-language processing approach that got shelved because my boss felt it emphasized the skills of mathematicians more than his own, or to LARP rather than put in another weekend on my approach using principal component analysis for early cancer detection that it turned out some guy from the FDA had already published 6 months earlier, or the technique for choosing siRNA sequences that a professor from George Mason already had a paper in press on - all those times, my brain was using a better estimate of success than my reflective self was.
Another question is why the "important stuff" is important. Fun is fun. On the surface, we are saying something like, "I have a part of my utility function that values contributions to the world, because I evolved to be altruistic." If we really believe that, then for any contribution to the world, there exists some quantity of fun that would outweigh it. And people use language like, "WoW may be fun, but it has little lasting effect." But when you contribute something to the world, if the relevant motivating factor to us is how our utility function evaluates that contribution, then that also has little lasting effect. If you do something great for the world, it may have a lasting effect on the world; but the time you spend feeling good about it is not as great - probably less time, and a less intense emotion, than if you had spent all the time accomplishing it playing WoW instead. So this question is about whether we really believe the stories we tell ourselves about our utility functions.
1. He got to award himself all of the department's yearly bonus money that wasn't awarded to his subordinates, so any obvious success by his subordinates was money out of his pocket.