Nick Yee has an interesting take on addiction factors used by game desginers in MMORPG. To put it shortly, there seems to be 3 main attraction factors of MMORPGs that encourage time investment and personal attachment:

the elaborate rewards cycle inherent in MMORPGs that works like a carrot on a stick. Rewards are given very quickly in the beginning of the game. You kill a creature with 2-3 hits. You gain a level in 5-10 minutes. And you can gain crafting skill with very little failure. But the intervals between these rewards grow exponentially fairly quickly. Very soon, it takes 5 hours and then 20 hours of game time before you can gain a level. The game works by giving you instantaneous gratification upfront and leading you down a slippery slope. And it overlays different reward cycles so you're always close to some reward - whether this be a level, a crafting skill, or a quest.

the network of relationships that a player accumulates over time. There are several reasons why relationships of a platonic or romantic nature occur so frequently in MMORPGs. The anonymity and computer-mediated chat environment facilitates self-disclosure, and many players have told personal issues or secrets to online friends that they have never told their real life friends or family. The high-stress situations inherent in the game also help build trust and bonds between players very rapidly. Of course, another important reason is that the games were designed so that you have to group to achieve most goals.

the immersive nature of these virtual environments. This factor works by encouraging players to become attached to their characters and the virtually valuable items that they own. The immersive nature also encourages players to become personally invested to what happens to their characters, and to be empathetic towards their characters. In the same way that a movie or fairy-tale enchants you, the immersive quality of MMORPGs tries to enchant you with a fantasy, and make you feel that you are part of something grand and extraordinary.

He gathered these information through a research process detailed here in the context of his Ariadne project.