Programming something into an MMO is only part of the problem. You can't just code in things willy nilly though.

For example quests.

It's easy to create a quest. But you have to think of balancing the rewards. How much the rewards will effect the economy. Will the quests create a different leveling flow than the game is currently designed for. Will the gear rewards effect crafting. ( will no-one craft *similar* items, for example. )

That's just off the top of my head. Plus that's only for something like simple quests.

God knows how hard it must be to implement changes to a fundamental part of the game. The Battle system.