LinkI'm curious about how important having every underground feature is to people.
I remember when it was limited, I would usually look for either a underground river end, or a chasm. I looked for these because I liked having fancy waterfalls in my fortress. I didn't need 4 or 5 features. And as I played, I stopped looking for those features too. And a short time later we got our undergrounds in their current state.
Right now I use the underground for it's different raids. I make my fortress connect to the surface first, then connect it to the underground. I do this to increase the chance of a game-ending raid without triggering the deepest HFS.
Ultimately, I don't need a ton of features. Connection to a cavern layer that allows the HFS is more then enough, and that only so we have the option to be besieged in a few more directions. For adventure mode, a fortress alone is a point of interest. More people go to see Mesa Verde(561,163 in 2006) each year in Colorado's 4 corners than go to see New Mexico's Carlsbad Caverns(407,367 in 2006).
I'll put my current fortress up as a candidate for exploration over anything I've found in the underground currently any day. If it doesn't add anything to the design of my fortress, it's presence underground isn't needed. If it does add, it's presence explains partially why my fortress is there anyway. So make them rare for adventure mode. As long as we can see them, we can place our fortresses where we want to take advantage of what we need to. That way adventuring will remain more fun. But on that route, I am unhappy with the current interface between adventure mode and lost fortresses...