To me, from experience, the #1 is the IGE's of the world. Those people tend to really trash the economy over time, and are disruptive in the game world.
Take WoW for example. If you check the auction house's you'd usually find most epics and a lot of the nice blue's bought out by the farmers (Just about everyone on whichever server knows who they are for that server). They then reprice them, usually, astronomically high to force you to buy their gold. That, or you have to farm yourself for a long time to acquire that gold.
That in turn makes you run into the farmers/bot'rs IN the gameword who try to monopolize the good areas and force you out by any way possible. (Usually they train you, try to kill you if on a PvP server, etc).
It gets so bad its sickening.