Agree with Overspeculated on many of his points. According to a Dwarf Talk, I think it was, Toady, too, has considered the lack of a goal in DF. The original Sim City had scenarios in which you had to clean up after a power plant going haywire and exploding, repairing Dresden after WWII, turn a sleepy village into a bustling city, reduce pollution in Detroit, and so on. DF, while I don't see it having anything like an Age of Empires system where the scenario ends when you "win", could have "missions" you get from your hometown. As Toady says, the fortress must've been set up for a reason, people don't just go into the wilderness saying "let's make a huge mountain hall here!".
The good side to DF is that unlike the pitfall of monotony that was The Sims, there's a whole lot of things to do. It's not just "get everyone out of bed, give them breakfast, send them off to school, watch them come home and entertain themselves, give them dinner and put them to bed, rinse and repeat". There's just so many challenges that it boggles the mind. Still, it would be good to have story/objective arcs or some sort of "main quest" that went with each fortress or just evolved more or less randomly (or based on the events around the world) over time.
As for UI, I had the same impression - downloaded DF, didn't get it (especially what with it being in alpha), and uninstalled it. Then I got back into it some time later when I found the afteractionreporter tutorial.