Gameplay
Gameplay in Minecraft right now consists purely of constructing and destroying blocks around a massive ocean. There are several different types of blocks, some which perform special functions.
With these blocks, many players create large constructions such as castles, roads, buildings or skywalks, along with other forms of artwork. A chessboard or giant picture rarely goes amiss on a server.
Not only have users constructed buildings, art, and caves, but also mini-sports. Using the current ingame features, users have been putting them to clever use. An example of a Minecraft sport is Spleef.
Notch is planning on adding a Survival mode, Zombie Siege, and perhaps other gamemodes, to give it more gameplay. Right now it's considered a sandbox construction game.