Well, maybe you have; it's almost always mentioned on "recommend a game" threads on Reddit, despite it still being in alpha, and even despite the interface being rather… scary. And there's a good reason for that.
Dwarf Fortress is, as the name implies, a game involving dwarves and fortresses. Specifically, you make said forts for said dwarves. It's a bit like the old classic Dungeon Keeper in that regard.
But don't let the website or the graphics fool you; it may look like something out of the 1980's, but DF is very much a 21st century game, with breadth and depth that surpasses pretty much anything. Even a medium sized world probably has more impressive procedurally generated content than an entire Spore galaxy; realistically flowed rivers which have carved the landscape, lava flows and volcanoes, glaciers and deserts, oceans and aquifers, lakes and caves, mountains and ravines, layers and veins of ore, rock, gems and soil. All populated with creatures of every description, some of which you can tame, eat or milk, and many of which that'll kill you given half the chance. Some you can trade with, others will siege you, or steal from you, and you can do the same to them.
The world even procedurally generates its own *history*: civilizations battle each other, individuals get wounded (limbs removed, fingers broken, organs skewered, blinded, burned, frozen, drowned, smashed…), get married, have kids. Artifacts get made, often decorated with art which reflects events in history, megabeats roam the lands, heros kill and get killed by them, and indeed they get injured too (meaning you might get attacked by, say, a legless dragon, or a hydra with only 4 heads).
And into this world, you send your dwarves; you can choose almost anywhere to settle. Forested valley with a river running through it; haunted glacier with a cliff face and magma pipe; desert with an underground lake; mountain with a waterfall; coastline with beaches; wilderness; barren island.
And so, you start off with seven dwarves and whatever you choose to bring with you; a wide range of meat, plants, seeds, weapons, tools, drink, animals (which can be adopted as pets), etc. When you arrive, you start to dig out your fortress (or build it out of materials you find or brought with you), making stockpiles for your food and other stuff, barracks or individual rooms for your dwarves, workshops to make things like beds and doors, floodgates and levers and mechanisms to build water systems so you can build a well (for when your dwarves can't drink the alcohol you're brewing in your still). Farm plots so you can grow your seeds, butchers to turn wildlife or your tame animals into food and materials you can turn into crafts (to trade with caravans for things you need). I could go on and on.
And of course where you embark has important implications for what you can do and how you do things; get magma and you can process metal ores and forge metal items without wood or charcoal, and if you have sand you can make glass. If you don't have wood you need to bring and trade for some to make beds and burn (especially if you don't have magma). On cold maps, water will freeze, so you need to get some indoors so it's available to drink; on a glacier you may need to dig out a section and collapse it to make a small reservoir. Screw up, and you might end up with injured dwarves who need to drink water, but without any water to give them, erk. And you did save some wood to make a bucket with, right? An aquifer might lie above the rock in the region; you either need to find some rock that goes through it, or build pumps to remove the water so you can make walls to keep it out so you can dig through it. And of course the pumps need power; be it wind, flowing water or just dwarfpower. Of course, without rock you'll be severely constrained in your available construction materials to make these things…
And yes, flowing water; the game has a bunch of simulated liquid flows: water (including pressure; many a fort has been lost when someone placed their well on a lower level than the water source), magma, steam, mist, dust, miasma (from rotting items), fire, smoke. Even temperature is modelled, though I'm not sure how much of it is a flow.
Dwarves themselves are modelled in a lot of detail too. They have personalities, and they can mesh or clash, making friends, enemies, lovers, etc. They have likes and dislikes, and moods; the fortress, environment, people, and events can all make dwarves happier or unhappier, and they can get so despondent they become suicidal (jumping off cliffs, or down wells, or into rivers or magma), or become violent. Consider: one of your soldiers has had a baby. They carry it around with them. (S)he gets into a battle, and wins, but the baby gets killed. Said soldier becomes depressed, and finally goes insane, killing his wife, his other children, his pets, a member of the fortress guard, and finally he kills himself. And if any of them were close friends with other members of the fort, well, you better keep a close eye on them…
Dwarves even make art; artifacts and engraved walls often have images on them relating to events in history, or things that have happened in the fort, or even just things the artist likes. Did that guy jump off a cliff? You might find an engraving on your walls of a dwarf flinging himself into the abyss. Did you repel a siege without loss? You might find your next artifact is decorated with an image of a dwarf and a terrified goblin, with the dwarf laughing at it. You might even get images of other artifacts or other masterworks. Or you know, you might just get yet another image of a $!&* door.
And of course, there are endless ways to lose. And no real way to win; winning is just not losing. It's a sandbox game, and you can do whatever you like until you die or abandon the fort. Thankfully, losing is fun. And afterwards, you can reclaim your fortress and try again, or visit it as an individual adventurer to see what became of it after you all died.
Now, it's not all roses and fluffy kitties; yes, it looks like The Matrix (but graphics sets help, e.g. May Green's excellent DFG), but more critically, the interface is rather… preliminary. It's almost entirely keyboard driven, and the keybindings are not always entirely consistant. Large constructions are tricky, even before you worry about making it so dwarves don't get trapped or crushed (the latter is at least a bit realistic, but it gets old once they wall themselves in for the 5th time), and there's so much depth, it can take a considerable amount of effort to grasp what you need to do to even survive. Which I guess is also fairly realistic.
Still, if a lot of people can find the unremitting shallow repetitive tedious nonsense of Spore exciting and fun, I can find the unremitting depth and complexity of DF exciting. And call people who like Spore names.
Now, if you'll excuse me, I have six dwarves on a glacier to see to.