Major turn offs:
* Anywhere where the damsel is terrified. It just doesn't work for me at all. Absolutely kills all interest in the scene. Unfortunately, that makes at least half the World's quicksand material useless to me. Each to his own.
* Masturbation: I don't know these ladies even slightly. I'm just uncomfortable watching something so private, even if it is all fake. Weird huh?
* Incompatible age: if she looks young enough to be my daughter or old enough to be my mother, again, no interest.
* Unwilling sinker: if I get the sense that the actress isn't really into it, I feel sorry for her, instead of enjoying the sinking.
Other disappointments:
* thin, definitely-not-gooey-in-any-way fake quicksand
* fake struggling in something that's not thick, or half-hearted 'clawing' to get out. We can tell, you know!
* messy face - no thanks.
For any possible aspect of this genre, I'm sure there will be some people who find it essential, others who can't stand it.
As a counterpoint to an otherwise negative post, I love the exploring scenes, both amateur and actress. The 'first time' scenes can be great too (cautious is fine). Plain no-plot sinking is fine. Anything where a fit gal goes readily, slowly, surely into deep, thick mud

KS
PS In response to Billie (below), it is quite possible to be sexually aroused by the deep mud experience without involving masturbation (fingering of genitals). That's just not something I'm keen on seeing. Regards reasons for getting into mud: the main reason is just that if feels great, but there are other less immediately sexual reasons for deliberately getting into the mud, such as: exploring; rescuing; construction/conservation/agricultural work; testing/renovating a mud pit or a new area; athletic training; sporting event. None of these are fearful, but actually for me they don't really work that well as a plot device for getting the leading lady into the mud: it's difficult to realistically reconcile the plot with staying in the mud. In the end I prefer no plot, just sinking because it feels so good.