There are key differences in progression as well, between the games.  Linearity vs bubbles can change the culture dramatically, with more area specific preferences occuring in the latter.  As well there are the issues with Instancing, spawn timing, dungeon accessability, key requirements, gear dependence, strat dependence, stat dependence etc are all details i am ignorant on to some degree when it comes to other MMOs.  It is those details that can shape the desirability of the game, and to whom it is desirable.

A game where progression is open and based on capability attracts definition powerplayers.  That is, if you can enter the final dungeon at any point in the game without a key, but only be strong enough to actually fight there once you have farmed the content of the second hardest dungeon, and so on.

If its more about time investment, and not necassarily the power of the creatures one can kill for loot, then a more social, RP style base will develop.  That is, If to reach final dungeon you need a key randomly dropped off only NPCs from the second last dungeon, and so on.

But that is only assuming one single variable.  Other styles can drastically change the atmosphere.  Loot styles effect economies, economies effect farm/camp styles, as do NPC availability, like open timed respawn zones, or instances for just you and your party.  Each of these variables can effect the demographic of the playerbase, and conflicting influences can often be to blame for an MMO being very popular, or failing to secure steady sales/subscriptions.

So i guess, to sum it all up, only one who has played through every corner of every game, and able to have enjoyed both the extreme power play and RP styles of play, can be a strong witness to the defining natures of MMORPGs.  Man, i just totally managed to oust myself of a coherent point.  Dude, did it just taste purple for a second there?

Having led an endgame EQ1 guild for 5 years, powerplaying has its perks.  To keep 72 ADD retards raiding in symphony for 5 hours a night, 5 nights a week,  was a rush in itself, destroying our targets was bonus for me (though the only bonus for many, stories for another day).  One infamous dungeon took over 6 hours to clear with 72 people, and that was 6 hours of intense concentration due to respawn rates and mob difficulty.  Many stuck it out for the comradery and fun, but many were lootwhores.  The way loot worked in EQ1 allowed for much influence and drama to occur.

WoW in comparison is nothing.  Its simple and accessable for anyone with thumbs.  Its MMO for retards. 

I never Tried Guild Wars, but had heard many good things about it from people i knew for years that burnt out on EQ1.  That microcosm of comparison meant a lot to me, but playing it will be the only way i can know for sure.  I would go and pick it up, but i try to only do one MMO at a time, and PS has me rather enthralled, even though unfinished.  God help me if a raiding atmosphere develops here, my wife will hate this game as much as she hated EQ.

Fiction:
The Fountainhead - Ayn Rand.  While Anthem was a discussion on the soul of society, the Fountainhead is an observation of the souls of men.
The Dirk Gently Series by Douglas Adams, woefully underated due to the success of HitchHiker's guide
East of Eden - favourite Steinbech
The Dark Tower Series - Stephen King.  If you dont care for the style he has been typecast into (supernatural horror and gross outs) this is the series that almost his entire library is sourced from.  An epic tale devoid of Stephen King's fame making nastiness.
I heartily recommend you avoid Dan Brown.

Non-Fiction
The Hiram Key, Second Messiah, and Uriel's Machine - Christopher Knight and Robert Lomas's lifelong work's research into Freemasonry's origins and history.
Earth Chronicles Series - Zacharia Sitchin, one of the original "Aliens created Man" theorists.  His attention to detail and depth of research and reference material is what gives this read the meat to finish its wordy trek through his theories.
All Over but the Shoutin' - Rick Bragg - the only "grew up the poorest of the poor in the deep stinky hot south with no pappy and a mama that never stopped working to keep us in clothes but i still managed to get out and make something of myself" that really ever touched me.  He had me at the opening paragraph of the prologue, and i never shed so many tears reading a book before, or since.
Swing Low - Miriam Toews - The sad story of a father with undiagnosed depression as a vessel to illuminate the secretive, protectionist nature of Mennonite culture and lifestyle.
Again i have to recommend that you never read a Dan Brown book, since he refuses to officially declare his work as fiction, and as a bastard child of his imagination and opinion, i felt i must place this warning, in the non-fiction setting as well.