It's weird that nobody came yet with the super original answer "500 points! ^^". I guess their differences are more or less similar that the ones there are between 1300 and 1800 players (http://www.chess.com/forum/view/general/whats-the-main-difference-between-a-1300-and-a-1800-player), though I would say that there are more differences between 1700/1800 players than between 1200/1300 players.
What is the difference from a 1200 and a 1700?
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What is the main difference between a 1200 and an 1700 in chess?