If you take 10 million random people and sort them by height, you'll get a bell curve called a Gauss curve, or "normal distribution": There are very few very tall people, some tall people, lots of average height people, some small people, and very few very small people. The reason this is called a normal distribution is because most natural distributions look like this. And although it is much harder to measure, if you sorted the 10 million World of Warcraft players by skill, you'd get the same distribution. 7ou'll get everything from low skill to extremely skilled, with the large majority being somewhere in the middle. Only the very low skill end of the curve is missing, because people who don't have enough skill to even do one quest won't play very long.
And the same is true with every other game. People have different skills in video gaming. And if your skill isn't high enough for the minimum skill level needed to succeed, you don't play. I suck at first person shooters, so I don't play them. Which means my dollars are lost to the game companies making first person shooters. So Blizzard, clever as they are, made World of Warcraft very easy. Many people who have difficulties to succeed in the average video game still manage to kill monsters and do quests in World of Warcraft. And the learning curve for WoW isn't steep at all. You get new abilities every two levels, but most of them aren't any harder to use than the first two abilities you started the game with. If you managed to kill that wolf at level 1 to gain your very first experience point, chances are you'll also be able to kill that level 69 mob that makes you ding 70.
But shooting for the lower end of the skill curve has one disadvantage: Soloing in World of Warcraft in most cases isn't very exciting if you are of average skill or above. If a game is too easy, it isn't challenging enough to be fun. That is why so many people rush through the leveling game and try to get to the level cap as quickly as possible, even paying other people for power-leveling them to there. At the level cap you can group, and you can raid, and while you still don't need to be a rocket scientist to successfully group or raid, you definitely need more skill for that than for soloing. You need to watch what the other players in your group or raid are doing, and the combats last longer, forcing you to think about concepts like mana efficiency or regeneration.
Now if World of Warcraft was much harder, it probably wouldn't have gotten 10 million subscribers. Being accessible to everyone is a strength. But I wonder if WoW wasn't even better if it offered more challenge to players for who it is too easy. Sure, you can fight monsters of higher level than you are even now. But why would you? The fights last twice as long and don't give twice the xp, so in the end you'll level slower if you search the challenge. There is no solo content which would give better loot for higher challenge, like a group dungeon does. And groups at lower level nowadays are hard to find, if your time schedule would allow you participation at all. I'd love to see solo instanced class quests, like Age of Conan has in the lower levels, introduced to World of Warcraft as well. They could even be designed to teach you your various class skill, by having challenges you can only overcome by using your class-specific spells and abilities.
Or of course the expansions that add levels to the game could have those levels be more difficult (and not just longer) than the previous levels. Unfortunately I don't think Blizzard is about to do that. I'm looking forward to Wrath of the Lich King for the exploration of the new content. But I doubt soloing up to 80 will challenge me, or most other WoW players.