Interesting addition to the OP. There's a monumental amount of evidence that our brains are biased in that way. It's called addiction, akrasia, procrastination, etc. An example is when short-term indicators such as how something tastes mis-align with long-term ones like whether later you get a stomach ache, nausea, dizziness, etc. For example, you may eat a donut now because of how great it tastes, but then regret it later because you end up feeling sick and nauseous.
I consider this a modern world problem. I assume that all these indicators are supposed to be in harmony, but the modern environment breaks this delicate balance. We end up with a conflict of interests among our different selves over time. Perhaps the most famous and familiar example would be night guy vs. morning guy. It would be one thing if you went to sleep too late one night, found yourself miserable the next day, and then made sure to not make that mistake again... but it's another one entirely to do this systematically--day in and day out.
This phenomenon of the internal conflict between night guy and morning guy is another indicator problem. Your signals for going to sleep mis-align with those of whether you got enough sleep by the time the next morning morning rolls around. This conflict is intractable. Your night self wants to stay up, but it's not him that pays the damages; it's your morning self. What's the cause of this internal conflict? Artificial lights, unnatural lack of exercise, comfortable chairs, TV, computers, the internet, etc. Once again, the modern world breaks a delicate system.
So what about WoW? Well, is staying up super late unmitigated enjoyment? Is refreshing LW and your email 85 times per hour as fun as your revealed preferences would suggest? No. They blow. There are many downsides. The LW refresh cycles that last until 4 AM usually include their fair share of FML. Fun is fun, but it stops being worth it when "FML!!" gets too interspersed between those moments of fun. Or later when you find that you've lost your health, your girlfriend, your job, etc.
I find that every kind of super-stimuli (e.g., junk food, music, WoW) has a slew of disadvantages. They do damage to your health, your social life, your work, your emotional sanity, and so on, whether directly or through causing you to neglect other areas of your life. If these things don't matter to you, then fine. But I really doubt that. Being healthy, happy, etc... that is fun. Going emotionally insane, being sleep deprived, losing your health, not getting any face-to-face social contact... that blows almost no matter how you slice it.
If you can play WoW in moderation and enjoy it and not feel any bad cognitive effects or hits to your health, that's great. But it's very addictive in that it will send an overwhelming shock-wave through the rest of your natural reward system, and your brain will start equating the health of your character with your own. This is scary shit. Your identity will become unstable, and looking at yourself in the mirror after a long WoW session will be a bizarre experience. This might well advance until the day you admit to yourself that you've gone completely insane.
WoW is not mind safe. Again, I wouldn't touch it with a 10-foot pole. I can get raw enjoyment out of things that won't threaten to destroy my health, drive me insane, wreck me emotionally, etc. And many of those things will build a whole slew of skills that I will be able to use to have even more fun in the future. WoW may well build some important skills, but the downsides and cognitive hazards are just too massive.
Interesting addition to the OP. There's a monumental amount of evidence that our brains are biased in that way. It's called addiction, akrasia, procrastination, etc.
The hypothesis I'm suggesting is that all of that evidence is only evidence when evaluated from the giddily-optimistic view of "what I could have done with all that time if I hadn't wasted it". Not from comparison with the accomplishments of a control group that didn't waste their time.
...I can get raw enjoyment out of things that won't threaten to destroy my health, drive me insan
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.