NPCs being indistinguishable from living beings in adventure mode (natural language, true intelligence, procedural, plausible motivations) - in other words, you should be able to cary on a conversation with an NPC via text and have them react with 100% realism, and this would affect their future interaction with other NPCs. NPCs also converse and interact with each other in a similar manner. NPCs remember, learn, and grow as individuals in the same way as humans.
You can't make a lava tank out of ice or wood. Or a house out of ash. Or a bridge made of soap that's a kilometer long.
Smashing something under a bridge doesn't annihilate its atoms.
Flying creatures don't need a ground path.
Pets can be butchered.
Animals can eat fast enough to avoid starvation.
Dwarves develop a self-preservation instinct.
Dwarves discover that fire can be put out with water.
Dwarves stop being obsessed with laundry.
Battle is dangerous. Consult with your squad leader if you are pregnant or nursing before fighting demons.
Traders are no longer interested in garbage.
This is a legendary statue. Press V to view.
This is a masterful mural by Thikolangelo. Press V to view.
This is a masterful composition by Nadost Beethoven. Press L to listen.
The mountainhome demands to know what is becoming of the immigrants they've sent, and why your outpost hasn't been sending quarterly reports as agreed. A fact-finding expedition has been dispatched to investigate.
The recent death of Baron Thikud has shocked the Dwarven population. They demand justice. A detachment of Dwarven peacekeepers has been deployed to police your fortress.
Your massacre of Elven traders has become an international incident. Your fortress has been sanctioned by the United Kingdoms. A multilateral force of Elves, Dwarves and Humans will arrive at your fortress next month to assassinate you and institute a leader more receptive to democracy.
You have discovered steam power! You have developed calculus! You have discovered electricity! You have discovered the turbine! You have discovered the germ theory of disease! You have discovered general relativity! You have struck uranium!
I don't actually want technology, but it would be neat as an option.
Almost forgot: an interface that doesn't suck.