Tobold has made some good points, and I wanted to address them. A lot of the disagreements on the Morons & Slackers discussion are emerging from miscommunication. So - I wanted to write a brief overview on some of the major player types in WoW.
Casual: This is the most common type of player. A casual player is one who plays for the sake of it. He expects nothing out of the game, except to have a simple good time. He plays for leisure. He may have short-term goals, like "get to this next level" or "do a dungeon tonight with friends," but nothing too in-depth. Casuals can be both good and bad players.
Slacker: The slacker is almost only different from the casual because of his motivations. A casual seeks nothing more than his own in-game efforts can provide. A slacker, on the other hands, expects more out of the game than he is willing to work for. Most slackers are not good players (though they could be).
Elitist: An elitist is one who sets high standards for himself (he also tends to expect everybody else to have the same standards, and may judge by them). Elitists are often the best players, and are the opposites of slackers. Note, however, that an elitist is actually not that different from a slacker - what separates them is that the elitist will actually work for the goals he has set.
Moron: Based on the definition provided by Tobold (mentally disabled, IQ below 70), then yes, there are most likely no morons playing WoW. In fact, "moron" does not fit with any of the other categories.
Take a look at the first 3 player types. They are based on behavior and expectations, and say nothing about one's intelligence. Calling somebody a "moron," however, is passing judgment on one's intelligence. But is it? Look at the context. When somebody repeatedly stands in fire and blames the healer for dying (or does not even realize it is the fire that is killing him), do we really think the person behind the computer has an IQ below 70? Do we really believe he is a drooling mental retard? I doubt it.
We are calling the behavior of the in-game character moronic. Standing in fire is stupid. There may be many different real-life factors to explain why that player stands in fire. Most likely, that player is a real-life slacker. But in the game (from an RP point-of-view, if you will), his character is a moron. And it doesn't take an IQ below 70 to be bad at a game.
So my main point is that Tobold is right (at least partially so): there are most likely no morons playing WoW. However, there are morons in WoW.
EDIT: There is also the issue of age. A 12-year old with an IQ of 140 will still have less maturity, knowledge, and common sense than a 30-year old player of average IQ.