Adult Legos in your own massive, unique, emergent world.
It is worth noting that the game is buggy, as expected, and that it's not free; but pre-ordering the game now only costs $13 and I feel better about this purchase than most if not all of the $50-70 games I've bought in the past.
The gameplay involved surviving in a wide, wide world. During the day there is only the threat of "Creepers," enigmatic and silent ticking time-bombs. During night and in the caverns, however, lurk arrow-shooting skeletons, moaning zombies, and giant hopping spiders as well.
Why should you play Minecraft? Notch is an independent developer who, through a simple and brilliant game has made a million dollars with which he's going to reinvest in his own company. The game is updated (almost) every Friday and is only getting better. You can even play online with your friends, and experience which is unparalleled and which is receiving constant support and bug fixes.
Really, the only reason you'll need is to search "Minecraft" on Google images.