Great discussion all around, as I was hoping for... or rather, expecting.
Just thought I'd tweak this a tad with a look at the quicksand itself and its role... as well as its consistency, make up and its look.
First, the age old debate over peat versus clay. I really think that is a matter of taste on all sides, and I like both, truth be told.
Now, I prefer peat for the more dramatic sinks, because it looks evil, being all dark and sinister like the better Hollywood pits out on the moors and jungles... Primarily, it seems more likely the heroine would miss it, because it would blend seamlessly into the dark forest floor, and our actress would then find out too late what she is knee deep in is a deadly trap. The pure white clay of earlier MPV videos seems to telegraph "deadly trap" so much you wonder if the lady was blind not to see it - efforts to coat it with dirt notwithstanding. (I mean no offence to the pioneers of qs vid, BTW... these were from the earliest scenes, and I loved them.... I'm just spoiled now...)
The darker clay works a lot better, again, in my opinion, as it could be mistaken for a mud puddle or mud bank by a river, and again, our heroine either stumbles in without knowing, or charges head on and is already in it before she knows what is happening.
Both peat and clay react nicely to the ladies, and in different ways, adding variety to the sinks. Always appreciated.
Now, I've wondered if it were possible to "gussy up" either with leaves or something to make them "harder" to spot by the approaching victims. I understand about contaminating the pits, but would there be something which would fit the bill, not be too expensive, which could simulate leaves or not be too organic, and thus decay? Just throwing that out there.
With dry sand sinks starting to arrive, very impressive I must say, that opens another idea... does one prefer dry and sandy, or dry on top of the bog, or just wet and sloppy from the get go? I like all three, personally, but it does depend on the scene. Those with lots of struggling kind of need that wet slapping sound as the victim claws at the bog. If she's not wiggling a lot, I think dry sand works just fine. But again, that's me.