Difficulty Curves

The two Portal games illustrate the idea of a difficulty curve beautifully. The game revolves around a series of tests that the player's character must pass. While the purpose of the testing is never made clear in the game, from a design standpoint they're clearly intended to teach the player to use "portals" to solve increasingly complex puzzles. The puzzles begin as very simple tasks, introducing the player to cubes that can be carried, buttons that can be held down by the player's weight or a cube placed on top of it, and the portal. As the game progresses, the player is given control of one, and later on both portals. The player is introduced to new elements, such as a deadly ball of energy that must be guided into a switch to progress through the test, walls on which no portals can be placed, and deadly robots. Every new obstacle is introduced slowly, so that the player is able to understand how to overcome that obstacle. In later levels, the same obstacle is used in new ways, sometimes combined with other obstacles, to produce a more difficult challenge. Every level builds on the previous level, teaching the player a new skill and then forcing them to apply it to a novel situation.

For an excellent example of Portal's learning curve, the 7-minute video below shows a walk-through of the level where hostile robots are first introduced to the player. As the level progresses, the robots are placed in move difficult positions, forcing the player to come up more more creative strategies to get past them.