Minecraft is deep
Let's start with the basics here. Minecraft is an indie game developed by Mojang Specifications with a focus on two things: mining and crafting. If you play the single-player mode as I have, then you've found a very brutal experience. Minecraft drops you into the world and literally says, "Go." You're left to your own devices in this cute-looking title where everything is made out of blocks, even the sun. It's like you're standing in a giant love letter to 8-bit games of yesteryear.
As you wander around, you'll soon realize that you can punch and break stuff. Punch the dirt and you get blocks of dirt in your inventory. Punch the sand and you get blocks of sand. Punch the trees and you'll find wood, and it's the wood that makes things interesting.
Placing wood in your crafting panel creates lumber, and placing in two blocks of lumber creates four sticks. Sticks are your first building block in this game, and they can be used to form rudimentary tools like a pickaxe, shovel, wood axe, etc. Soon you'll find yourself digging through the dirt and stone, creating mine shafts like a real miner. Each block you find can be used in different ways, like creating a wooden workbench or using stone to create stone tools.
Of course, you could skip all of this and set Minecraft to "creative" mode (you wuss), where you're free to place any type of block you want, from water, to lava, to even solid gold. But let's assume that you didn't do that and you're playing the single-player mode... and the sun is starting to set.
Minecraft is survival horror
The pitch-black night brings new challenges. Combining sticks and coal creates torches, and you'll need them to fend off the horrors of the dark. Spiders, skeletons, zombies, and the dreaded creepers (evil green monstrosities that explode when they sneak up on you) want to make your life a living hell. Once again, your workbench is your savior as you create swords, armor, and other weapons to keep them from killing you. Death means you lose all of your items to the world. Sure you can run back in and pick everything up again, but that means braving the monsters without all of your tools, if you're not careful.
But monsters don't just lurk on the map at night. Mining down into the depths of the earth is dangerous, as you may come across underground caves or even dungeons that are filled with monsters. You'll hear them groaning and scratching, and you'll wonder where they are. You'll venture into the darkness and explore with your torches, only to be surprised as they sneak up on you and scare the everloving shit straight out of you.
What was once a fun, cute world has turned into a game that can make you jump out of your chair.
Minecraft is beautiful
But even with the horrors that lurk beneath (the odd, blocky, pixelated horrors), you'll find beautiful vistas, amazing underground caves filled with their own rivers, lava flows that light up the darkest depths of your mine shafts, and ore that can be scavenged to make everything from mine carts, to boats, to electrical lines. Minecraft is your world, and no one else's.
Unless, of course, you elect to take the game online and play with your friends. Then it can turn into an MMO-esque building exercise, in which everyone participates in building amazing structures.