WoW has alot of things beside gameplay that makes people still buy it and play it and never really losing thier customer base.
first off, its Blizzard, and on top of that, Warcraft has had a HUGE fanbase for a very long time now. they can also get alot more players because its an easy game to pick up and play, you dont really have to know what your doing in this game, alot of just pressing buttons and jumping back and forth through characters, unlike FFXI where if you didnt know what you were doing, you could get killed and everyone else in the zone (mass trains).
and also the fact that its both PC and Mac compatible helps. and the requirements arent as processor intensive as FFXI. Low rez poly characters, shitty UV layouts but great textures in WoW. FFXI was really detailed.
and if you know a bit about computer graphics, blizzard didnt use a smooth IK/FK rig bind, they used a rigid and mainly animated using clusters, which makes it shitty(er) animation but less to code, thus less system requirements.
i could go on all day on the differences, but i dont feel like it.
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FFXI - Yurichan: 75THF(Ret) / SC2 - Lawncare / XBOX - Zombie Sharks / PS3 - Zombie_Sharks