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Machinima released a great 4 part video about the development of MMO's
This was indeed an interesting watch for me and I am sure a lot more people will enjoy it.
Part 1: "Crawling Through the Mud": http://www.youtube.com/watch?v=PgYuJczGv8o
Part 2: "Expansions": http://www.youtube.com/watch?v=s9IVlCE89bk
Part 3 "High Level": http://www.youtube.com/watch?v=GnR9P_dqINg
Part 4 "End Game Content": http://www.youtube.com/watch?v=WDXorHNo3eE
Now stop all the "WoW Clone" stuff as it is obvious that WoW only used what was successful in MMO's before it.
I am sorry if this was posted before, personally haven't seen it here.