Spiderweb Games - Using Our Powers for Evil

Jeff Vogel's latest View From the Bottom column at RPG Vault looks at gaming design that is primarily intended to create addiction - and MMOs are the obvious example:

Everyone knows that World of Warcraft, its sinister older cousin EverQuest and similar games are addicting. But what is the exact quality they possess that makes them that way? Is there a specific property they possess that we can extract, distill and pour into other games? And, if we do this, are we using our powers for evil?