The secret to designing great games. While this is the Diablo II review, it's undoubtedly relevant to most other "Crack Games" such as EverQuest. Anyone know of any scientific research around this subject matter?

"The actual gameplay still consists almost exclusively of killing monsters to gain treasure and experience points. Since your character constantly gains more and more formidable abilities and weaponry, that relatively simple style of play proves to be just as addictive as it was in the original Diablo and in other games that have since exploited the same formula. It's difficult to extract yourself from a game that always keeps you on the verge of being rewarded for another achievement. "

GameSpot: PC Reviews: Diablo II Review