Creative problem solving and STEM!
Students are creating something. They're not just playing a game to learn, their creating a game to learn and teach others. They're having to think critically, and because their games are published, they have to learn to take and give constructive criticism.
I have found that some students get frustrated, but in the end they take great pride in their accomplishments.
How I Use It
I use this with 5th grade students in my computer lab. They are able to work individually or collaboratively with peers. There is some direct teach, but they also go through "Quests" to learn how to create a good and "balanced" games.
Students also use Gamestar to tell stories or educate others with their games.