There is something compelling about making a bit of game content and inserting it into the world. You might sell it, use it, or give it away. It's yours. You made it and it had your name on it. I've never been happy with any of the crafting systems I've tried, but the crafting in WoW keeps a lot of people amused. This is one area where you can best WoW if you're willing to put the time into it. People want to create interesting and visually appealing items. Giving them the same level of control over the stuff they make that you give them over their avatar will make people want to make items not just for their intrinsic value in gold or damage output, but for their artistic value as well.
The amount of variety in WoW is staggering. The different sides, races, classes, and areas of specialization within each class mean that there is always something new to try. There are layers of depth that let players build their character according to their own concept or play style. You need to offer your players lots of choices that lead to lots of different gameplay. If you start with "do I use fireballs or a sword?" then you're thinking too small.
Some people get addicted to Ebay. The wheeling and dealing gives them a thrill, and then there's the joy of having new stuff appear on your doorstep. The auction house in WoW can offer that same experience, and many players enjoy making money by buying low and selling high. They shop mid-week when prices are lower and then sell their goods over the weekend when demand increases. They track drop rates and usage patterns to get a feel for when things will be needed. The auction in WoW is superb, and functions as an economic day trading mini-game for people with the right financial disposition. Make sure your auction system is up to standards, or your world will be filled with people spamming chat in all caps looking for buyers or sellers. This breaks immersion, interferes with the social layer of the game, and frustrates the people trying to do business.
There aren't many MMO games that don't have player-versus-player action in some form or another. It's an activity, and a crucial one. Many players build their characters for the express purpose of destroying other characters on the field of battle. Other people build their character because they enjoy the process of building. You need to let both of these people have fun without cannibalizing each other's gameplay. The PvP player should be given an outlet where they can meaningfully compete for rewards, and the builder should have the ability to opt out of the ganking and corpse-camping.
It's true that a vast majority of MMO players are filled with epic levels of apathy towards the story in the game. They click on a quest dude, do the quest, and run off again without reading a single line of prose. But WoW has robust (by MMO standards) storytelling and the NPCs generally try to make it worth your while if you take the time to read what they have to say. You might be tempted to cut corners here, but a lot of players enjoy knowing that they inhabit a good story, even when they're fuzzy on the specifics.
Enough MMOs have staggered into the fray before they were ready and ended up turning hype and anticipation into bad press and ill will. I would think that their failures ought to be enough to convince you to complete and then polish your MMO before you hit the launch button. You can only launch once.
Far too many MMO games will simply lock onto two or three items in the WoW activity list and decide that, "this is what WoW is all about." They build a game around those simple elements and neglect the others, and then wonder why their game is an empty wasteland while the population of WoW is more than double that of the entire country of New Zealand.
People dismiss MMO games as "grinding" or a "leveling treadmill," but the really successful games are more like a health club with a broad selection of exercise machines than a single treadmill. Yes, there is a lot of repetition, but you can jump from one (repetitious) activity to another to keep things interesting. In the broadest sense, the activities in these games stimulate the part of your brain that enjoys creating order from chaos by getting stuff done. Only instead of doing useful stuff like the dishes or your homework, you're doing stuff like cleaning out rat infestations, gathering up resources, and building items. In the real world, the war against entropy isn't nearly as swift or as gratifying. The stuff you're getting "done" starts as short-term goals that lead to longer-term goals that lead to really long-term goals. You're always on the threshold of getting something done and getting your next reward. (And your next goal.)
It may look like players just stand around killing monsters over and over (because they are) but that's not the heart of the game and not the source of motivation. Give the player lots of goals and you'll go a long way to making a game they never want to quit. Then sit back and let the cash roll in, you greedy, life-sucking, career-ruining, homewrecking bastards.
Be sure to send me an invite when you reach beta.