Server load- The server needs to be able to handle the game. If the world is too big, or graphics are to detailed, or too many things are happening at once, the game could slow down.
Balance- Balance is very important in games. If some action is too good, then everyone will start using that, and if something is not good enough, no one will use it because it is seen as useless.
Economy- The economy must also be considered when creating the game. Economy affects how well players will aquire items and trade with each other. Inflation and supply and demand are real issues in games and poor planning can ruin a game's market and eventually destroy the game itself.
Players' needs- Players opinions are important as well. If players find something difficult to understand or unneccessarily hard to use, they will start complaining. Of course, not every player idea is a good idea, but sometimes players come up with excellent new things.