The hardest part in an MMO ...

[monster]

Link of interest for aspiring future game devs ...

Most problems in creating a large-scale game world have technical, proven solutions, if you read the right blogs/books. It is not secret knowledge.

But there is one thing for which no perfect cure seems to exist, so far: inflation

Here just one more of many examples: Hyperinflation in a Video Game World

Preventing inflation is a real problem in such game, that could very well result in most players leaving, causing the game to fail. While most indie devs will not have such issues with their games due to the smaller scale, it is still a problem to be aware of, if you plan to grow, because it's really hard to solve, without loosing your face, once you do have an army of players, who have already invested real money in the game, and might perceive your changes, targetted at getting the inflation under control, as "stealing them". A game company is not the government; players are free to leave any time if they don't like your decisions, and without players, there can be no profit.

Second hardest, IMO, but less financially devastating, is achieving a "game balance", while at the same time permanently introducing new content. After each content upgrade, the game balance must be adjusted, which is tough, if you're not a "mathematical analyst".