If people are happy with how it is then that's coo.l This game is feature rich and honestly feels complete as it is, it's great that toady wants to add new stuff but I personally just want to play the game in a stable state where I can have all the features as intended. Does no one really feel this way?
Of course we all want that. We want it all, and we want it now. But, as they say, you can't always get what you want.
What if the game had been open sourced back during the 2d -> 3d conversion?
It's really hard to say. I have no doubt that it would be more stable-- people have been making fixes via hacks that still haven't been implemented officially, and that's without code access.
But I think it's important to realize that open source programming means losing a single, creative authority. This is especially true with something intended to be as procedurally generated as Dwarf Fortress; there's little real division between code and content. If it was open source, whips almost certainly wouldn't behave the way they do-- and we'd still have a philosopher and guild nobles-- and iron would probably be much more effective than bronze, more so than it is now. It's hard to imagine all of the creative changes that would come with open sourcing DF, and yeah, some of them might be for the better. But I believe that single visions lead to more distinctive, unique works than crowdsourced efforts. Yeah, Wikipedia is great. But so is
The Devil's Dictionary. They're great in different ways.
Any group that wants to is free to make an open-sourced DF-a-like. The source material is not so distinctive that you'd ever be vulnerable to a lawsuit. If you strongly believe that an open source DF-like game would be superior, then get it started. If you don't want to get it started-- because christ it's a lot of work, right?-- then maybe you can understand the immense amount of effort Toady has put into DF, and maybe begin to understand why he might be loathe to relinquish control of it.
EDIT: A few more things I think would be different if DF was OS: No forgotten beasts. Different methods of figuring temperature and vegetation, for good or ill. Vampires as civ leaders, never as infiltrators. Wider variety of vegetation. No evil weather.