Crafting in MMOs is a complex and widely varying thing. While most games include crafting in some form or another, it doesn't seem like there's much of a consensus on what purpose crafting is supposed to serve, and similar systems are often received radically differently depending on the title in question. Brian Green of Psychochild has had crafting on the brain recently and simplified the perceived goals of crafting into three areas (which I have further simplified): fun, utility, and money sink. His analysis is pretty in-depth, but he stops short of offering his own version of a crafting system, which we would have liked to have heard.
It's interesting that something as valuable to a rewarding MMO experience as crafting is so routinely put on the back-burner. Age of Conan and Tabula Rasa are both glaring examples of recent releases whose crafting systems are either incomplete or mostly useless as they exist in the game. We'll be interested to see whether Warhammer Online's crafting system will be as interesting as it seemed when it was explained to us, or whether it will join the ever-growing pile of time wasting duds.