The world also has a huge effect on your fortress. It's entirely possible, for example, to generate a world where every civilized race is in decline or extinct. How world gen goes determines what megabeasts appear at your fortress, how strong they are, and whether or not they are notable enough for your dwarfs to engrave their visage on your fortress walls. World gen creates the background to your fortress' story. In fact, sometimes the story of world gen is much more impressive than anything the player can create. Take, for example, the story of
Tholtig Cryptbrain the Waning Diamonds. The last queen of Bronze Orbs, she led her dwarfs into glorious combat against the filthy elves. In the end, she was the only dwarf to survive the war. Many attempts were made by the elves to storm the grand halls of Circletowers, where her throne sat deep beneath the mountain. Every attempt was rebuffed by the warrior-queen, who single-handedly defended Circletowers for ten years, before finally dying alone beneath that mountain. She died of old age, seated on her throne. It was never taken from her as long as she lived.
There's some embellishing to that tale, of course, but the point I'm trying to make is that world generation is what makes an otherwise fairly average convoluted management simulator, and makes it into an epic fantasy game. And, with the coming release, world generation is going to matter even more, because it sets up how the world will advance and change as you play. If the humans and the elves are having a big kerfuffle over the treatment of the Elves' marijuana patch, you might get refugees from a human civilization showing up on your doorstep. Or maybe some Elven diplomat makes one too many short jokes at your king's expense, and he launches a war against the tall, impertinent bastards. The second one, I don't know if it will be in this next release. But I imagine Toady would want something like that in the final game. The political machinations of far away cities always seem to come back to bite the outlying villages in fantasy stories.