The game mechanic is based around survival, and an open world. You are free to effect change as you wish, to suite your needs to survive in game. To survive you must gather limited resources just to live through the first night. As you progress into the game, you become more secure in your survival, so you can risk more to establish yourself more. Eventually the mechanic is that you've ended up with concrete changes upon the world you play in, can take pride in them, and then expand on them. Emergent gameplay, where non-scripted events happen, making your experience unique provides a great deal of re-playability. Additionally since Minecraft is under active development, I think a lot of people are looking at the game with an eye towards it's ultimate potential in their mind.

See http://en.wikipedia.org/wiki/Open_world

Also http://en.wikipedia.org/wiki/System_Shock_2 http://en.wikipedia.org/wiki/SimCity_4 http://en.wikipedia.org/wiki/Dwarf_Fortress http://en.wikipedia.org/wiki/Little_big_planet

Also http://www.youtube.com/user/davidr64yt#p/c/E005D335B57338D1 - A play through of the game from a first time users perspective

http://www.youtube.com/watch?v=DS3s4iVijpA - Minecraft Calculator

http://www.youtube.com/watch?v=f6JqoE7MZEE - Minecraft 16bit adder using sand.