Open-ended world building.
Joran: Create a thriving community, starting with 7 dwarfs. There is no ending, so any fortress will fail eventually. Their slogan is: "Losing is fun".
Wiki link here: http://en.wikipedia.org/wiki/Dwarf_Fortress#cite_note-gamasutra-2
A rich world generation;
History of your world recorded as lore;
Dwarfs digging too deep;
And wow. Look at the royal chambers that a player have designed.