I think what turns away players is that the game is both quite vast, content wise (food/goods production, monster attacks, and just keeping dwarves alive were the challenges that most concerned me in my first fort) and it simultaneously lacks any kind of direction. I love sandbox-style games, but without any kind of initial 'tutorial' or guidance, your first fort is almost doomed to fail. (fun fact: mine didn't until I reach the 2D version hidden fun stuff countdown, but the friend who got me into the game told me how to build a farm, which is all you really need)
I think either a tutorial segment (like the game coming with a pre-gened fort in a psuedo world with scripted hazards and events and a walkthrough dialog), or a more improved UI, would do wonders toward helping new players tackle Z-Levels, Farming, Creatures/Sieges, and Trade.
DF is a massively complex game, and any improvements that make the game more intuitive make the game better for new players. (For example, when I started, I hated navigating the labyrinthine build menu, so I memorized most of the hotkeys. Build a bed? B-B. A Butcher's Shop? B-W-U, which I memorized as standing for for "bUtcher")
Aside from that, a way to 'win' might help new players who want to work toward a goal. I know a lot of requests have been made for a way for your fort to be 'saveable' so that once it reaches a certain point, it counts as self-sustaining and you could abandon it, run around in adventure mode, and then even resume the fort without reclaiming. Having a win condition like that does not mean the player has nothing more to work toward (there is still the need for a magma-waterfall, or in the upcoming versions, to conquer the other races), merely that they can earn a bit more freedom to explore the game.