Here is an article from a couple of User Experience researchers / designers who have studied the emotions behind our needs for playing computer games and a written a White Paper called Why We Play Games. User Experience research is a derivative from the Human-Computer Interaction discipline which focuses on studying the design and use of interfaces. If you have a feeling that all this has to do with psychology somehow, you are right: the research methodologies and principles underlying this area of study borrows heavily from psychology (whether it's a cross-discpline or actually a subdiscipline of psychology is my own personal debate). Anyways, have a read. Note how Everquest is mentioned in ALL Four Keys to More Emotion without Story (Hard Fun, Easy Fun, Altered States, The People Factor).

This is what makes me ponder a bit, taking from the company's page on this paper:
"Forget Usability! What Makes It Fun?

As Player Experience Researchers, we see a huge gap between most usability and market research reports on gaming and what players actually experience. As Designers, we also see a huge opportunity for game developers to spawn player emotions beyond frustration, excitement, and fear. While players may check "good graphics" on a survey; our cross-genre, contextual research on why adults play games reveals a much more interesting story.
...
XEODesign,® Inc. helps companies create compelling interactive entertainment. We have spent over a decade researching software usability and more recently how interactive products trigger specific emotions. Our cutting-edge research and design services have improved the Player Experience for over 40 million customers of industry leaders such as UbiSoft, LeapFrog, Sony Online, Roxio, and Maxis. Our unique XEOAnalysis™ methodology measures emotional reactions during gameplay."

Wow, I wish they would spend that amount of time studying and designing a chair that woould evoke all these positive emotions in me...