The interface adds a lot of illusory difficulty, but that's not an issue for people used to reading manuals. On the other hand, when you're not wrestling with the system, there's not much fair challenge. When your underground fortress is filled to the brim with traps and completely self sufficient, no one is ever going to touch you unless you handicap yourself.
Once I had my base built out the way I wanted it, there wasn't much left for me to do. Kind of reminds me of how Minecraft feels once you've built a satisfying shelter, but I think that Minecraft's music and FPS perspective can motivate people to keep playing for the sake of creativity. With DF: sure, it's fun to imagine my fortress and all, but then I might as well move away from the keyboard and just do the whole thing in my imagination.