First I want to specift this is about the WoW type or MMORPG  and not Diablo type.

Everyone has their own grip about the MMORPG genre and the crop of games coming out lately. I don't think anything is wrong with any of them in particular, I think its the whole MMORPG formula that is flawed.

Let me define the general formula of any MMORPG - its quite simple: never ending grind.

These games are all based on that same basic formula. Each new MMORPG that comes out attempts to do one thing and one thing only: keep its playerbase in an eternal grind and attempt to gain a loyal player base while they're having fun.

In general those who quit a MMORPG are simply getting tired of the particular grind offered by that MMORPG. Tthere are other reasons, but when the player base of a MMORPG drastically drops by half, it isn't because they all lost their jobs at the same time :)

Every time a new MMORPGs comes out with some new "kewl" element, all they're doing is finding new ways to entertain the player while keeping him/her in the endless grind loop.

They use several "tools" to achieve this. Some are enjoyable, others not so much.

Enjoyable tools:

1. A leveling proess. Whether they call it "levels" or "skill up" it is the same thing: the player climbs a virtual ladder and becomes more and more powerful. I think most players would agree this is often the most enjoyable part when unlocking new abilities and learning the mechanics of the game,
2. Fat loot. Who doesn't like a good drop?
3. In game lore: some people like to know the story. For some players this is a big value, but others don't care one bit. This is a draw used mostly by big known IP like WoW,  Age of Conan, Star Wars, The Matrix, The Edler Scrolls Online, etc.. One has to explore hard to reach places, dungeons and always be on the lookout to find all the lore (at least until it can all be found on a fan page or other)
4. Crafting: again, this system only caters to some players.
5. Socialization. I know some people prefer to play solo - I am generally one of them. But why do you think they play a MMO to play solo? Because even if they play solo, there are other enjoyable elements such as PvP, chatter, trading, and the occasional good friends you can have.
6. Customization - have a unique look and stand out.

1. Repetition. By their very nature, MMORPG must have repetition. The repetitions are meant to keep you busy while the developers work on adding content. Hopefully they get the new content out before players tire of the repetition and leave the game. All enjoyable tools listed above slowly lose their appeal after so many repetitions. Every perosn has what I call a different "repetition cap" - a point at which they can't take it anymore. Mine is very shallow.
2. Hardcore players who play 10+ hours a day must be mixed with casual players who play 3 hours or less per day. The hardcore crowd is annoyed by the casual players who doesn't play as well and the casual crowd is annoyed either because they can't keep up with the hardcore or because they think the hardcore players are cheating (usually, they simply have a better grasp of the game mechanics).
3. Monthly fee for some
4. Purchaseable items for others that are required for players to be on par.

• SWTOR tried voice over: very immersive at first. Gets in the way after a short while.
• TERA tries fast paced and engaging combat and short miniskirts. Meh
• RIFT tried "rifts" and open world events. E-Z loot... *yawn*
• WoW tries to dumb down the game more and more so it gets easier and easier to get the loot... oops, now its too easy.