I unlocked 1999 Mode for Bioshock Infinite, and I'm a little curious why they chose to call it "1999 Mode" instead of "Very Hard" or something like that. What was so special about 1999?
It's intended to be a difficulty that hearkens back to the "hard old days" of FPS'es, as Ken Levine explains:
The sense that I get from reading multiple interviews with Levine is that he views the "modern" shooter as having been made easy for the masses. They wanted to give the hardcore player a chance to play a more challenging, strategic game.
They hid the 1999 Mode option (either by entering the Konami code or playing the game through on some other difficulty) so that the average gamer wouldn't stumble on it by accident.