The use as semantic stop sign comes from the fact that if something is obvious it does not need to be stated.
There are times when it's useful to state things that everyone present already knows. Most of the uses of "it's obvious" fall out of these cases.
For example, the rules of formal argument in an academic context might normally require each step to be given in full; but for pedagogical purposes it's not always useful to do that. The phrase there means you're temporarily lowering the level of rigor to make the argument clearer or to save time, and that's a useful thing to have in your toolbox.
Alternately, if you're making a complex argument with a lot of moving parts, salience issues often come into play; your audience might need to be periodically reminded of certain simple facts, but stating them without qualification might be viewed as condescending. "It's obvious that..." in this case serves as a social lubricant, a way of saying "I know you know 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?