Mainstream (FPS in particular):
Game. On. Rails. given that those rails pass beautiful vistas and are nicely set to avoid running into a wall or coasting effortlessly.
Dwarf Fortress / Roguelikes:
Game rolling end-over-end in the middle of a world where you can hardly see, sometimes you enter a flat, smooth plains, sometimes there is a giant friggin' grand canyon in your path.
Mainstream:
Giant monster approaching! It is about to smash the village! Quick, activate the photonic cannon and call the player a hero!
Dwarf Fortress:
Giant monster approaching! It is about to smash the village! Quick, Think of something or we are all doomed.
Mainstream:
A carp attacks! It deals 2 HP out of 100.
Dwarf Fortress:
A carp attacks! There where no survivors.
Mainstream:
Shallow learning curve. Not much to learn, much to master.
Complex mainstream:
Steep Learning curve. Lots of minutia to obsess over.
Very Complex un-mainstream modded:
Learning Cliff, ever tried to figure out what all of the stuff in Minecraft Redpower does (I know most of it, I think)? What about what all of the numbers mean in Kerbal Space Program FAR (what is a IZZ Derivative of sideslip angle?)?
Dwarf Fortress:
Wait, You expected something to be there to climb? Yeah, right. Learning Invisible Floating island... on another planet.
Mainstream:
Crisp, clear cut storyline and rich pre-programmed side-storylines.
Minecraft:
What Storyline? Make one yourself? What is that, you need characters? have a Squidward or ten?
Dwarf Fortress:
Story? How about you make a world, and the real interactions between characters, other characters, and the environment will be your storyline, background, etc.
Mainstream:
I have been disarmed, but I still have my trusty iron ingot to smash over their head! What? it only does the same damage as hitting them with my fist?
Dwarf Fortress:
That's right, I killed that Elven son of a ***** by throwing his own leg at him!
Mainstream:
Dead bodies are real physics objects that you can sort of collide with!
Dwarf Fortress:
Dead bodies are real food objects
that you can also throw parts of at people!
Mainstream:
Look at this amazing combat, headshots, leg-shots, arm-shots, chestshots, abdomen-shots! Each deal a different amount of damage and take a different amount of time to recover!
Dwarf Fortress:
Our combat system has unit icons...
It also has livers, spines, eyeballs, teeth, ribs, nerves.... The list goes on. What happens if your arm flies off? That should be obvious! Your arm lands somewhere and rots there.