I tend to compare Minecraft to Lego sets.
Usually, games are basically predetermined models with a background story (like the normal LEGO series. Ice Planet, Space, Blacktron, M-Tron, Exploriens, UFO... I stopped following the news after UFO). Of course, a PC game isn't as flexible as a LEGO playset. YOu can't just decide that BLackspace are the good guys, just because they have the more awesome base (COme on, it was pretty awesome for its time, all those moving parts... *sigh*), you're stuck with the predetermined story of the game.
Minecraft, on the other hand, is pretty much like getting a whole bunch of unrelated blocks and parts with no assembly instructions. You get the tools, you get the material, and you start building.
I personally love LEGO, but can't really work up a lot of motivation for Minecraft, so the comparisons might not really be good. Minecraft is still missing the physical interaction with the blocks.