At first glance, Minecraft is a game. But there's far more to Minecraft, as anyone who's played it will tell you: the first few moments of gameplay can make it obvious how much the game has to offer. Rather than static environments, Minecraft offers resources and tools. With those tools, users have created dramatic constructions: detailed replicas of famous buildings, large and elaborate machines, and generally monumental undertakings.
The beauty of Minecraft is that it's a reflection of human creativity and sheer engineering ability: for example, this tutorial page shows users how to build a 32-bit computer out of "redstone, torches, and repeaters leading into sticky pistons or redstone lamps..." unless, of course, redstone is very limited, in which case "using sand/gravel and signs" is advised.
Microsoft is experimenting with an AI agent by dropping it inside the world, much like the player is, and asking it to learn to perform tasks. The design team has spent time trying to get an AI agent to learn to walk uphill and learn for itself what kinds of factors matter to the task. Because Minecraft is so complex, it gives researchers ways to create environments, situations, and challenges that aren't offered by other research platforms:
The Minecraft platform was especially appealing because it allows players to make really complex decisions that have consequences, and to add more and more difficult elements as they get better. It also lets users work together, which could help researchers experiment with how humans and artificially intelligent agents could work together.