This article is a general primer on how to get players to continuously play your game, which in the world of the MMPORG is a very important element, using a behavioural model.

Gamasutra – Features – Behavioral Game Design.

This article is a bit more cynical and compares the above techniques to addiction creation and illustrates the theory with specific examples from World of Warcraft.

Cracked – 5 Creepy Ways Video Games Are Trying to Get You Addicted