It's not just the Lego aspect. There are two sides of the game which don't get enough credit: the aesthetics and the game mechanics.
By aesthetics I mean the combination of the graphics and the random terrain generator. They create some beautiful landscapes which are a pleasure to build on. In my first go at Minecraft I built a "house" out of an above-ground cave I converted, filled in and added doors to. Dug out the ugly dirt bits, replaced them with stone bricks and lit the place with recessed torches. Hollowed out the top of the mountain, added some large windows and watched the sun set over the trees. Pure joy. It's not just about what you build but how it fits into the world. (Which is why those giant pixel-art creations are very impressive but IMO miss the point a bit.)
Secondly, the game mechanics, the balance between effort and reward. The start of the game where you punch trees until they break is the butt of many jokes. But once you've got wood you can mine stone, once you've got stone you can mine coal and make torches, once you've got torches (and some weapons to defend yourself from the monsters) you can go underground and find some iron, and on and on it goes. It's a classic progression, like the Civilisation tech tree or the RPG get-bigger-sword-to-bash-bigger-monster routine.
The creative aspect gives you the incentive to progress through it. If I want my house to have iron railings I need to go underground and get some iron. If I want to set up something electrical I need to go further down and get some redstone. If I'm bored of torches for lighting, I need to create a portal and go to the Nether to find some glowstone. And so on.
Although most of the praise for Minecraft goes to the Lego-building aspect, there have been games that allowed you to build stuff out of blocks before (e.g. Infiniminer). Minecraft is a work of genius because of the way it marries that with beautiful aesthetics and balanced, rewarding gameplay.