Minecraft is a first-person fighter/sandbox construction game created by Mojang AB.
When I first showed some of my friends Minecraft their first impressions were along the lines of "All you do is place blocks in some world with crappy textures". Almost all of those friends now own the game.
Yes, the game is all about placing blocks, yes the textures are low resolution (16x16px infact), but it grows on you quickly.
What you build is limited purely by your imagination (and time). In a randomly generated terrain that is 8 times the size of earth there is no shortage of space to build in nor resources. Notch has some awesome terrain generation algorithms that can produce stunning mountains forests and other beautiful vistas, despite the fact that they are made up from 1 meter high blocks.
I'll say this:
If you don't already own Minecraft, give it a go - play it at a friends place or something and you will quickly see how fun it can be.