I have been thinking a lot lately about MMORPGs, and have personally played some of the stranger ones. Wurm Online, for instance, has your avatar do virtually everything in a semi-real environment where things take semi-real amounts of time. For example, you must dig down to rocks, prospect for ore, mine the ore, start a fire, melt the ore down, attempt to forge an item, fail or succeed, then do it all over again. To build a simple house you have to fell countless logs, make nails, etc etc etc all while keeping your person fed, watered, and healthy. Seems a bit excessive, but wouldn't be bad if the servers weren't so horribly laggy and it wasn't a java-based client.
Additionally, there are games like Entropia Universe, which convert cash into game money and vice-versa; EVE Online, which facilitates the buying of in-game time and characters with in-game money; World of Warcraft, which continues to provide new and interesting forms of gameplay, such as aerial bombardments and vehicle combat, and then drives it into the ground with repetition; Star Wars Galaxies and it's complex crafting systems, SWTOR with it's proposed companion system, WAR with it's guild-capturable keep and RVR innovations...the list is nearly endless.
As I said last time, games are becoming more and more specialized, and I'm not sure if I necessarily like it, however, I am wondering how far the trend will continue in terms of MMORPGs. These games, by definition, should appeal to a large audience, and their success usually depends on that very fact. Wurm online, for example, has a very small player base because at best it's tedious, and at worst, it's very tedious. WoW, on the other hand, has broken subscription records and danced on their mangled corpses because it's core game still appeals to a variety of players. It has diverse gameplay with no required path, a player is only rarely forced to experience certain content (the Death Knight intro sequence comes to mind, but that was very fun to play).
The rule of a successful MMORPG seems to be diversity to a point. Too much diversity and you have a broken system that is impossible to balance without ripping apart portions of the game(See Star Wars Galaxies Pre-CU and CU for more information), too little content and you are a specialized game that has a small audience and little ability to proceed (like Wurm online, and several other niche games). The appropriate balance must be met between certain aspects that appeal to the community, and ultimately one must take precedent over the others, even if they are semi-integrated.
These aspects are, in my opinion: Crafting, PVP, and PVE. Now these have a variety of subsets, but those are the broad categories. A quality game incorporates all three, and seeks some form of balance to them. WOW's priorities fall in this order: PVE, PVP, with crafting existing in mostly a token form (although, some professions are very important with the latest expansion, namely the new inscription class which provides benefits that cannot be looted). A game like SWG pre-CU roughly balanced PVP and Crafting as primary goals, with PVE hanging on for dear life as it was mainly used for xp and loot for crafting and pvp.
The appropriate balance is key for a successful game. WoW's lackluster crafting system has certainly not done much to restrict it's subscription numbers, and they are slowly improving it and making it a more important part of the whole. A game that brought all three into a healthy balance would appeal to virtually all gamers, however, would also most likely seem like three separate games. Integration is as important as balance.