Fun is such a strange thing. Depending on who you ask, fun in Minecraft can include hardcore combat, sprawling city-sized build projects, roleplaying games, brutally difficult survival, co-op with friends, trolling strangers, modded Minecraft, vanilla Minecraft, and so it goes. Actually defining "fun" isn't hard on its own (something we enjoy doing, and/or that entertains us), but what makes one thing fun for some players, and just dull (or even anger-inducing) for others?
In each case, it boils down to what a given activity does for us. For creative builders, there's the challenge (and satisfaction) of large-scale planning, and seeing something immense come out of what is, fundamentally, just a collection of blocks. For people with a combat focus, adrenaline is a huge motivator: chasing down (or being chased down by) other players, testing who has the better reflexes and skill, touches something competitive deep inside of us. Even on a smaller scale, the simple pleasure of building a nice house, or just completing a crafting project have a response in our need to organize our lives, or overcome obstacles that we face, both in and out of game.
One interesting thing about games (video and otherwise) is why we engage in them in the first place, besides entertainment: in some ways, games are practice for real-life things. While it's true none of us have to hone our skills against actual endermen, encountering them can improve situational awareness, reflexes, and critical thinking. The same is true of many aspects of games of all types.
If you had to give it some serious thought, what do you think is the most fun thing about Minecraft (or any game), and how do you think it affects your life outside of the game?