Well it does already kill you no matter what in DF if you fall from like 30z
And it does already approximate falling at various angles or body positions via its random dice rolls for damage.
So beyond that, it's pretty much hardness that would matter. Assuming:
1) That the first thing you hit doesn't break, if it does break then the force used up by breaking it is important, since it will soak up kinetic energy helpfully without hurting you much. Not too relevant to DF.
2) If it's a fluid, then surface tension essentially acts as hardness. Falling onto water will hurt you from much lower heights than falling into, say, a pool of hexane would (though you would still find that unpleasant for other reasons...). Since DF only has 2 liquids though, and one of them presents much bigger dangers anyway, this is also not super relevant.
Thus, pretty much just hardness, IMO. Which is basically the same thing you're getting at with "airbag" == artificially "soft" surface.
Imaging tripping and falling from just 6 feet or whatever your standing height is, and hitting your head. Would you rather land on cement, or balsa wood? It would make a HUGE difference. One might kill you or make you a vegetable, while the other might just give you a bad bruise. Though it's really the softness of the balsa wood, not the low density, that's helping you.
Falling onto lead, for instance, would generally be slightly safer than falling onto steel, since the lead will yield a tiny bit and thus distribute the force a tiny bit more equally. Whereas DF would simulate it the opposite way around, by a lot.