From the very first minute you spend in WoW you are put into a world that was created for you. Saying that may sound somewhat foolish I mean, of course the world was created for you, who else would it be for?
But after playing many different MMOs I have come to realise that the sense that you are invading uncharted territory, long forgotten crypts and ancient ruins is what makes these game fun. Enhanced by lore, simply gaining access to one of these areas is something lower level players aspire to. That's not to say that World of Warcraft does not have these areas, however they do not give the impression that they are pre-existing societies that you are attacking, it's as if they're just sitting in their lairs waiting to give you their loot.
Most of the problem has to do with the level of difficulty in WoW. You can level to 60 in a couple months the first time you play, after doing it once you can level to 60 in mere weeks the second time. I am a hardcore raider and I live for the high level game so being able to get to 60 so quickly is nice for me however when it doesn't require any real effort, social ability, or actual skill it really loses something.
When people don't have to put months or even years of their lives into achieving things those achievements become worthless. It's just like anything else in life, the more common it is the less valuable it becomes. Because gaining access to Onyxia's Lair, The Molten Core or even Blackwing Lair (the current hardest area) is so easy, easy enough that anyone with an afternoon and an ability to click "accept" to a group invite can obtain it; World of Warcarft is not a place where players are measured by their accomplishments. It is so easy to accomplish everything that it doesn't matter if you can kill Onyxia flawlessly with 25 or if it takes you 40 people and you have only 5 standing when she dies, all that people are measured by are the items that they have.
At the same time, when a group of adventurers who have gone from killing rabits to being able to kill gods and entities that have nearly dominated the entire world in only a couple of months, it really makes the entire game world seem trivial. Ragnaros, Onyxia and others that have usurped Stormwind and threatained the peace of all of Azeroth, beaten by forty young players who have barely had time to become attached to their characters just makes it all seem so fake.
It's really unfortunate that this is the case because WoW has the best gameplay mechanics I have seen in a MMO and to have the experience ruined for these two reasons is a huge letdown. When players no longer have anything to work towards the game gets boring. The reason EverQuest forged so many lasting friendships for so many players was because you could not accomplish anything for yourself without helping your guildmates or friends accomplish it too. Even if you have the best possible items and a key to every zone, there was not much you could do if you weren't willing to help others get the same.
EverQuest created an atmosphere where the guilds that were most co-operative and had the most selfless members flourished. In WoW the only thing that requires actual time and effort is obtaining faction, which is an entirely solo goal. Imagine playing a first person shooter where instead of playing through level to level until you reach the end and defeat the boss, you just play the same three levels over and over and after beating that level 200 times you will get a new weapon - doesn't sound like much fun does it? that is WoW.
There are many other issues that are causing high end players to leave the game almost as fast as they're joining, but that is what is on my mind. Speaking from experience with EverQuest, if loot is what drives you, you will get bored. In WoW there is nothing BUT loot to drive players, simply because everything else is so easy, everyone has it and it is therefore worthless.
When it comes down to it, it is all about the level of difficulty. It doesn't require the best possible items, months of gathering experience and keys, and making sure you, as a guild, have everything you need to take out a new mob it just requires 40 people who listen to instructions, and even the unskilled guilds can do that. Why would anyone play 10+ hours a day to get all the best items and potions, if you can do things without them? Why would anyone put in the time and effort required to raid every night and get items for people when those items are not even needed to "beat the game".
Blizzard tried to remove the gap between the casual and hardcore players, so that there wouldn't be so much content casual players could not access as in EverQuest, and they succeeded. What they neglected to realise is that it is the hardcore players that give the casual players something to aspire to, something to look forward to being, and when any casual player can get into the same areas, get better items purely by luck, and take out the same monsters sometimes without even fully understanding how they did it, the hardcore players are no longer hardcore, they're just a week ahead of everyone else, and that is not really worth the effort.