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Square enix made a statement that their sales were so high that it had to shut down their online sales of the game temporarily until the servers are given greater capacity to hold many more players on 9/2-9/4, and they are adding even more worlds.
You can read more on the details here:
I guess I would compare the success of FFXIV to the PS3 sales, where everyone was standing in line overnight just to get one. Fortunatly you were able to do it from home and no one got hurt!
The procuder even said in a letter the other day....
"the game has evolved into something much bigger than we ever thought possible."