In the bullet-point list of most every MMO on the market, a "functioning in-game economy" comes as a standard feature, much like AM/FM radio and power steering, er, I mean like PvP and spacebar-as-jump-button. Being able to buy and sell goods and the occasional service between players adds a layer of depth and strategy to the game world that -- for once -- isn't solely about killing something. A few select players even eschew the rest of the game to focus on playing the auction house or crafting goods to sell.
Unfortunately, like the Force and the moon, in-game economies always come with a dark side. Economies are tricky to balance, and the longer a game runs, the higher the risk for it to spiral out of control due to mudflation. Late-comers to the game often find themselves at an economic disadvantage in comparison to extremely wealthy veterans. Economies also need the support of a lot of desirable things to sell and a useful interface through which to do it (such as an auction house), creating a lot of work for the dev team. And, of course, with any economy comes the headache of gold farming and selling.
Out of curiosity, would you play a game with no real economy? Some games such as Free Realms
and Guild Wars
de-emphasize their economies greatly in favor of other parts of the game. Do you find MMO economies to be more of a bother than a blessing? If a fun game came out that lacked this "essential" feature, would it be a deal-breaker?