Looking it athe World of Warcraft Community, this just popped in my head. You can define different sections attributable to the addictive quality of an mmorpg. I'll try and break them down:

The social aspect.
Socialoisng with people in an anonymous environment. You can redefine yourself and make new friends

The comaradery.
The social aspect often leads to the forming of guilds when you are constantly together and watching each others backs.

The random drop effect.
The same effect gambling has. If you randomise the rewards, people will keep repeating the same actions over and over again. (the famous pigeon experiment)

The competitive aspect.
Often you'll see high levle players parade around a big city with their newly earned rewards. Commanding attention and lots of drooling fans (read: groupies) They then inspire people to go dugneon crawling themselves.

The growth aspect.
The feeling that you keep getting bettter and better.

The *something new* effect.
I forgot the scientific name for this, but people will always respond to something new. And since Blizard uses their newfoudn resources well and regularly updates the Word of Azeroth with new content, people keep coming back.

The trend effect.
Since so many are already playing WoW, they only attract more people. MMORPG's ahve been around for awhile, but Blizzard used their existing Warcraft fanbase and built a great game around an already cool lore filled story, which players have already experienced. This way they expanded their first-time mmorpg'ers and are constantly attracting more. (using good marketing and quality game engineering)

And ofcourse, the game is just fun to play, playable on a broad range of computers and extremely addictive.

See ya in Azeroth!