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Would you play a game with a thousand levels? I think the thing I like most about MMOs is leveling. Of coarse there wouldn't be new abilities all the way to 1000 but you'd get all your abilities fairly soon then they would just scale with you as you leveled.
I think it would take even the most dedicated player years to reach 1000, but the plus side is that there would always be something to work towards.
I think there are some games that have already been made with really high level caps, like Asheron's Call I think is pretty high.