I don't see how this is worse than any other probability estimate when a simple calculation with good numbers isn't available. Do you consider "probably", "unlikely", or "almost certainly" to be semantic stopsigns as well? Would it be better to say "very unlikely" or "with extremely low probability"?
When people act as if the probability of a certain event were zero but try to pacify the reader by using a fake zero called epsilon instead ("Oh, epsilon is very small but non-zero, so I suppose it's OK"), they consciously or subconsciously throw a semantic stop sign ("I don't want to be called on this").
I think most of us are familiar with the common semantic stopsigns like "God", "just because", and "it's a tradition." However, I've recently been noticing more interesting ones that I haven't really seen discussed on LW. (Or it's also likely that I missed those discussion.)
The first one is "humans are stupid." I notice this one very often, in particular in LW and other rationalist communities. The obvious problem here is that humans are not that stupid. Often what might seem like sheer stupidity was caused by a rather reasonable chain of actions and events. And even if a person or a group of people is being stupid, it's very interesting to chase down the cause. That's how you end up discovering biases from scratch or finding a great opportunity.
The second semantic stopsign is "should." Hat tip to Michael Vassar for bringing this one up. If you and I have a discussing about how I eat too much chocolate, and I say, "You are right, I should eat less chocolate," the conversation will basically end there. But 99 times out of a 100 nothing will actually come out of it. I try to taboo the word "should" from my vocabulary, so instead I will say something like, "You are right, I will not purchase any chocolate this month." This is a concrete actionable statement.
What other semantic stopsigns have you noticed in yourself and others?