To describe peroperly Minecraft is a difficult task. It can be described as "a construction sandbox", but it wouldn't make justice.
Think better of Minecraft as a digital version of LEGO. Think of Minecraft as what Oblivion would have wanted to be in terms of exploration.
The game presents with randomly generated infinite worlds. You can dig, hold and use objects, and craft things. At day there are animals, at night zombies and other terrible monsters (so you better hide!). Oh, and the world is full of different blocks/materials.
With this simple basics, an amazing world appears. Before I continue check this small but great trailer:
Surely there are other games that hook you for hours, but with Minecraft I feel like when I was a child playing with LEGO blocks and imaging them in motion, alive. This is the same but in first person.
Want to build a castle in an island? Go ahead!
I'm building mine :)
And it feels as rewarding as if you were really placing each block during the day, hiding from the monsters in the night. Days are rough, a pure survival until you are able to build a decent house/stronghold/whatever.
The world feels alive: Chickens leave eggs, animals can drown if you create deep water flows, lava slowly expands, you can see what time aprox. it is for the height of the sun or moon... And the crafting system has already a lot of objects, with some amazing things like electric circuits and logic gates.
Oh, and you can also setup your own multiplayer server. I'll probably setup one if any of my friends grabs the game.
Believe me, games like this make me feel there's still hope of innovation and quality on this gaming world full of crappy remakes or ultra-short games with "astonishing visuals".