Just from my personal experience, one of the things that makes MMORPG so addicting is that instant gratification. Accomplishing things in the "real world" typically takes a lot of work and struggling for a reward that could be years down the road, whereas in WoW all it takes is a little bit of time and a bit of pushing buttons and bam you've got that great feeling that you've done and accomplished something. That feeling is just like the greatest feeling, and you end up wanting more of it. The "real world" becomes less meaningful and these games just have so much content that it's nearly impossible to achieve everything there is to achieve. I had a couple projects with awesome potential that i essentially just cut off so that i could hit 60 or get some other virtual purple item, and all there is to do now is look back and think "what if". Of course this doesn't end up being the case for everyone, but it does for a lot people.
