Zoso linked to a post from Raph about how some games are akin to a 'treadmill'. Rather than post a World of Warcraft-specific reply to his site, I decided to expand my opinion a little and present it here.
[Treadmills] do not present variegated challenges nor truly new ways to interact with the model. This is why players get so annoyed at ?¢‚Ç¨?ìFireball VI?¢‚Ç¨¬ù: that?¢‚Ç¨‚Ñ¢s not a new stick to poke the beehive with, it?¢‚Ç¨‚Ñ¢s just a bigger stick with exactly the same properties and responses. It doesn?¢‚Ç¨‚Ñ¢t keep the user learning. Where the learning stops, the fun does.
The learning in a game like World of Warcraft does not stop, and there is skill involved all the way from level 1 to 70, and in nearly every instanced dungeon up to that point. The problem is that the unskilled players do not realise that there are new skills to learn. They may well see 'Fireball VI' and think that it's just a bigger stick to poke the beehive, when what they should really be seeing is that they are no longer poking beehives.
It's difficult to die before 5th level in WoW, and it's relatively easy to chain-kill enemies with nary a rest for many levels after that. But there comes a point where you are more threatened, where you need to recover after fewer and fewer fights. It's not because you are getting weaker as a character, and it's not because the enemies are stronger as such, but because the enemies are more sophisticated.
The enemies that are encountered throughout the levelling process keep getting more sophisticated, acquiring different attacks and different defences, just as a player does, but at a diminished rate from players. Because WoW has treadmill-like traits, it certainly is possible to use your Fireball from level 1 to 70 and succeed, but you are guaranteed to die along the way. The skilled player will learn enemies' behaviours, see what tools they have available both to themselves and the enemy, and adapt to suit the fight. This allows them not only to succeed, but to be succeed more quickly and as a result be far more capable at surviving. The skilled player will likely die on occasion too, but under noticably different circumstances than the unskilled player.
It seems that the real problem here is that players encounter a 55th level orc mob and see the same enemy as the 10th level orc they encountered ages ago, and fight it the same way. The similarities are almost as superficial as a 10th level human warrior and a 55th level human warrior: they have similar apperances, but they will perform quite differently. Encounter an enemy in an instance and they will have even more abilities, and it becomes even more necessary to learn the differences between enemies in order to succeed.
There is a lot to learn in a game like World of Warcraft over that of character abilities. Those players with less skill or aptitude will not realise this, but the forgiving nature of the treadmill still allows them to continue playing. The skilful players learn with each new enemy, and continue to improve their character without having to learn new spells or abilities. Both types of players get to learn, but at different times, and the balance between skill and treadmill in the PvE world allows both types of player to continue in the game world.