How I feel:
Walls: All materials should have some what of identifying how "hard" it is. Granite? Hard. Talc? Really soft. Then all walls would have a specific defensive value.
Then we can have things start effecting them. Okay, I can live with this. Walls can be beaten away. They could even have damage portrayed in text rather than a visible value(Scratched, Cracking, Furrowed, Crumbling, Breached, Rubble). I would love that. Of course, rubble would slow people down. This would make going through a thick wall made of iron ore even slower than normal.
Digging: I'm not in love with the idea, but I do realize it would be cool to battle goblins as they breach your main hall. Making sure your main hall isn't always a battle-site whenever a groundhog comes around would suck, though.
I don't know how cave-ins worked in the 2-D version, but I assume a solid block of earth drops down with a bunch of smoke made. Then you can mine the collapsed rock? The way I see it, when cave-ins are fixed ground can be "Created" by cave-ins. Of course, mining walls made by cave-ins only produces rubble, which cannot be made into anything except rough walls and slows down movement when stepping over it. These walls made from rubble would have a LARGE negative modifier to how much they protect. Also: Rubble can only occur as of the layer type. Found an iron vein in a limestone layer? Any cave-ins there would only produce limestone rubble.
This way players can create natural walls without creating horrible holes in the wilderness, yet can abuse it.
Personally, I want to see what Toady will do when he gets around to fixing support.