Building an expansive fortress and number of dwarfs is a lot of fun (honestly, probably the most fun I've had with a game since Dungeon Keeper and Settlers way back) but then FPS starts dropping and you find yourself doing dumb things to counteract that. The fun bits fade away and it gets tedious.
To me, DF is best looked at and enjoyed as an amazing art project and an inspiration for other game devs.
Edit: Actually, my evidence is dwarf therapist. I don't think i could play the game without it and it is evidence of two things: 1) the default interface is broken in some serious ways, 2) the game is so good that people will go to really _insane_ lengths to mod it to make it playable.
The community is in fact participating, but the creators are making it more difficult than it needs to be with all the source closing.
But regardless, its their pool and they choose how they want to operate it. We're fortunate that they share it with everyone because it is a very cool project, but that doesn't mean we can't take note that it falls under the weight of itself fairly quickly in practice.