First, I absolutely love Dwarf Fortress. There's only a few things that jump to mind on how it could be be better. First, it's pretty darn inaccessible to new players; they're thrown up again the intimidating "Generate New World", then the multitude of embark options, and when they finally get started, they're up against a wall of ASCII representing a wrecked Dwarven wagon, wondering "what the hell do I do now?". I concur with some other suggestions here: Have it come with a pre-generated world, with a good standard embark profile. Also, a tutorial would be immensely helpful. Perhaps a game-in-progress, specifically built as a tutorial using Notes. That seems well within the bounds of what can be done right now.
Second, the performance. Oh Armok, the FPS death. The biggest hitters seem to be pathfinding and item counts. Perhaps pathfinding can use a better algorithm, or otherwise be improved? Can items be handled more efficiently? Something more simple though, put in an options screen to change things in the init file. It'll appeal more to the less hardcore hacker types. Some preset options profiles that would do things like Weather: OFF automatically.
Third, and more of a personal thing than the others: it needs more difficulty in the latter tiers of experience. But I assume that will happen with things like the siege arc, so this is a much more backburner thing.
Those are the things that I think would improve the game the most and broaden its appeal. Again, I'm amazed. This is probably the greatest game ever made, and it's not even done yet. The only one to improve against now is yourself, so keep up the good work. Your fans regard you with awe because of it.