Completing the game once unlocks a New Game Plus mode where you retain all of your upgrades and items whilst the enemies behave more intelligently and can kill the player with only a single bullet or hand-to-hand strike.
The player's line of sight is limited to a cone aimed in whatever direction the player is facing (preventing players from seeing what is behind, above, or below them unless they turn to look)
The player is unable to see the enemy's line of sight and must intuit it from the direction an enemy's face is pointed, and the player can no longer detect the radius at which their actions will be heard by enemies.
Furthermore, all of the tutorial-like "cutscenes" which are meant to introduce new mechanics and/or enemies in the game are no longer shown in New Game Plus.