I'll comment again to note that I am struck by the relevance of Tom McCabe's post Levels of Action. Tom noted that we can think of some actions as "additively" improving the world and others as "multiplicatively" improving the world. Your choosing to take up WoW would definitely additively improve the world insofar as it's the most enjoyable thing you can be doing at the times you play it (whether that's true depends on a couple of other factors, but stipulate that). But it would almost certainly not multiplicatively improve the world—or even multiplicatively improve your total enjoyment of the world, if you were a pure hedonist—and that is the standard you should be applying when evaluating the opportunity costs.

(This should be read as a followup to my comment here.)