Affinity Groups
One of the fundamental advantages that the participatory culture of informal spaces such as games can bring to learners is that they provide opportunities to form affinity groups. In these groups, members share similar goals, values and motivations; as a consequence, they become powerful settings for learning.
Because both experts and novices in a domain usually constitute affinity groups, they allow multiple opportunities for novices to learn the advanced skills of experts. At the same time, given their eminently participatory nature, novices are required to enact (however poorly) the activities of experts in the field, thus giving ample opportunity for practice.
Videogames such as World of Warcraft embody these principles particularly well. A player begins as a novice and works alone through the early levels. As monsters become more difficult to defeat, he or she, with the help of expert guides, keeps advancing through the game.
Semiotic Domains
Another aspect that makes games so powerful is that players and designers engage in the pursuit of common meaning. In games, this is usually established by the rules the designer puts into the game. As the player interacts with these rules, the local meaning of the elements on screen gets validated or corrected as needed.
Let's look at a simple game like Pac-Man. In the game, you must collect round pills, large and small, which award points while avoiding the pastel-colored ghosts. However, only upon collecting one of the big pills does it's full meaning present itself: the "ghosts" begin to shimmer and run around in a frightened manner, thereby informing the player that Pac-Man can now chase and eat the ghosts.