Modern MMORPGs are multi-million dollar projects, and thus come with so many bells and whistles that we forget what these games are about. What I am going to try here is to look beyond particular games and explore the root purpose and means to achieve that purpose of a MMORPG. Thereby I hope to construct a simple model of MMORPGs in general, and see how features work or fail when compared to the model.
A game is defined as an interactive entertainment undertaken for enjoyment. In a MMORPG the interactivity has a social component, interaction with other players, and a more static component of interaction with the game itself. The interaction with the game comes in two basic flavors: repetitive and non-repetitive. A typical example for a repetitive interaction would be a combat. While playing a MMORPG you will have many combats, and while the exact details might slightly vary, the basic gameplay of combat is always the same and repetitive. Having such a repetitive component differentiates a MMORPG from other forms of entertainment, like most books or movies. The advantage for the game developer of having basic repetitive units in a game is that you only need to program them once and get many hours of entertainment out of it. But that is not to say that repetitive is bad for the players; for example combats being similar to each other allows players to learn how to do them best by trial and error, until they ultimately master the activity, which can be a lot of fun.
If you consider the basic repetitive units as the bricks of a MMORPG, the non-repetitive part is the mortar that keeps those bricks from falling over. We usually call this non-repetitive part the "content" of the game. Content is everything that isn't repetitive: zones, quests, or the game's lore. Doing just one fight after the other quickly becomes boring; but doing quests that encourage you to explore zones are much more interesting, even if that leads to repeated combats against the same type of mob.
And that is already the simplest form of the model: basic repetitive units like combat surrounded by non-repetitive content like quests. All the other stuff, gaining experience, leveling up, equipping yourself, collecting magical treasures, is all just virtual rewards, a Skinner Box to motivate us to keep playing the content and basic repetitive units of the game. And while anything with a chat function is open to an infinite multitude of social interaction, the social tools in a MMORPG are mostly designed to enable people to go through the basic repetitive units and content together. The goal is to keep you entertained, which is exactly what the player wants, but also to keep you paying for it, which is what the game company wants.
How well a MMORPG succeeds in keeping you entertained (and paying) depends on the quality and variety of the basic repetitive units, as well as the quality and quantity of the non-repetitive content. Quality is difficult: Everybody wants it, but nobody has a clear idea on how to produce it, except by throwing tons of money at the problem during development. Quantity of content is also money-related, because designing a good zone or quest takes development time, and providing lots of good zones and quests takes lots of time, thus costs lots of money. Cheap cop-outs like copying and pasting content, or randomizing the creation of zones and quests, have been proven to not work; you can't make the part of your game that is supposed to be non-repetitive into another repetitive part. It leaves you with too little content to hold the game together.
But seeing how much entertainment players get out of combat, a basic repetitive unit which is relatively cheap to produce, it is surprising how few games manage to add a larger variety of high-quality basic repetitive units. Instead of making for example crafting into an activity which is as entertaining as combat, and thus would keep people entertained for a long time, many games reduced it to a few simple clicks, which isn't entertaining at all. Adding other mini-games, like card games, to a MMORPG is still at a very early stage of development, a promising one. Whenever developers add features to a game, they need to consider how much time players can actually spend playing with those features. Why introduce a feature like player housing when all the house does is sit there? Having housing as just another money sink, without any play value, is a waste of development time.
So I think the future of MMORPGs is adding a larger variety of basic repetitive units which are equally entertaining. Instead of having a brick wall, where all the bricks are nearly the same, held together by the mortar of content, we get a stone wall with many different forms of stones. Players get a larger choice of activities to pursue at any given moment, which leads to repetitive features becoming boring less quickly. MMORPGs are already much advanced, offering a thousand and more hours of entertainment, instead of less than a hundred hours like a single-player game. But that still puts us on a cycle where few players play the same game for longer than three years. MMORPGs need to evolve further to break through that barrier.