I was thinking about what it is that makes Dwarf Fortress different or special. Or, a different game from everything else.
One unique element is the consequences passage of time. The game goes through 4 seasons, with different things to look forward to or beware of in each. Caravans, crops, migrants, diplomats, and especially weather. Each year brings you closer to more sieges. Population keeps growing the more time passes. You can't control time, you can only try to work faster and adapt to it. It's more than just a race against the computer. Sieges are like the typical "player vs. AI" game of most RTS.
Next is the buildup of responsibilities. You have to feed, booze, and clothe your population/adventurer or bad things will happen. You have to create things for your nobles, and obey laws. Having to feed yourself comes from roguelikes and old RPGs (Ultima comes to mind), but its more important in fortress mode where you have to take care of your whole population. Having a wide variety of plants, with seeds, and ways to use them seems unique. Maybe Populous/Black and White/Civilization had similar ideas? The whole system of stockpiles feels more flexible in dwarf fortress, though. You can be as prepared or unprepared as you want.
Lack of control over population is pretty unique, its not like the other games where you need a certain amount of food to grow... you'll grow first, and then are expected to make the food! Nobles will come whether you are ready or not.
Having a huge procedurally generated world is special for DF. Civilization was the first game I played like it, but the concept is taken to the limit in DF. Geological accuracy is unique to this game. And you can select your starting location!
DF has the best fluid simulation, and lets you use water for a variety of purposes. And magma. The games you can play with water are what I feel are the most unique parts of DF.
Individuality and customization of each dwarf is pretty unique. Everyone has a lineage, thoughts, personality, and separate stats and skills. It makes the world "alive", especially when these people can show up around the world after the fort is over.