Show him a Megaproject.
I think megaprojects kind of miss the point... To someone who hasn't already played DF, megaprojects in DF look like crap. You can do them in Minecraft more easily, and they look better there. Why bother slaving away getting 200 dwarves making something silly when you can hit the 'import' button in someone's minecraft mod and have it already done and properly rendered?
Not that I advocate that, but to someone who hasn't played the game, that's the key -- what makes it fun to do that, or anything, in DF?
I think the key things to point out are:
1) The game world is an intricate story which is never the same twice, all the way down to trading partners and wars.
2) The game is the struggle to lead your dwarves to survival and prosperity in an environment of your choosing.
3) It's a managerial game. Your interaction is even less direct than things like Simcity.
4) Combat is gruesome enough to make one grateful it's just ASCII.
5) You can play it in any style you want: you can be a benevolent dictator, or a crazy overlord who executes all children. You can focus on making your dwarfs life's happy, or you can just forget your troubles and love the magma cannon.
6) The random name generator: Nothing more needs to be said.
7) What I personally view as the most important: The community. I am personally inspired by what I see go on these forums (both the good and the bad). Deathgate inspired me to invade the Clown Car, Boatmurdered gave me a fear of elephants, and a deep appreciation of magma, and Doomforest reminded me that even if 199 out of 200 dwarfs die, you can still come back. It's not over till every dwarf is dead and haunting.
If this is the standard, I might change it to this.