Here's an interesting article about what makes video games so addictive. It made me think about the Skritter guys sitting around saying "Why isn't there a ninja video game that teaches us to write kanji?"
Some relevant quotes:
One ingredient is “hard fun,” which Ms. Lazzaro defines as overcoming obstacles in pursuit of a goal. That’s the same appeal of old-fashioned puzzles, but the video games provide something new: instantaneous feedback and continual encouragement, both from the computer and from the other players.
Players get steady rewards for little achievements as they amass points and progress to higher levels, with the challenges becoming harder as their skill increases.
Even though they fail over and over, they remain motivated to keep going until they succeed and experience what game researchers call “fiero.” The term (Italian for “proud”) describes the feeling that makes a gamer lift both arms above the head in triumph.
http://www.nytimes.com/2010/12/07/science/07tierney.html