What would consider 100% completion in Minecraft?

To complete the game you need to defeat the ender dragon, technically that is the end of the game as the credit sequence starts once going back from the end. As it is an open world sandbox game there's no end, the end is when you decide to give up playing or start a new world, you're in control of how the game runs.

Once you complete the achievements, and defeat the Ender Dragon, you are 'done. Except not really of course, as in Minecraft there's always something else to do.