For instance, many discussions of race in speculative fiction start with the assumption -- never, of course, laid on the table -- that the racial categories of modern day society are like the law of gravity.
Reading even Edgar Rice Burroughs or Robert E. Howard perhaps can clear up the notion, but then, I actually noticed the racial effects far better after I had been primed by reading about the racial science of their times. (The very, very, very best racial science of their times.)
The classic mix of elves, dwarves, etc. could help: groups that used to classify themselves all as different races often band together when face with people very much more different than themselves. It can, however, take a while. It took several centuries for the Indians to think of themselves as all "red" as opposed to "white" -- whom they generally split up at first.
And you seldom get the terminology that divides the world up into us, the real people/tribe/nationality, and them, everyone else, who get lumped together into one word.