We're not talking super-hard vocabulary or weird syntax here. The language in Wide Sargasso Sea is simple, but every word is weighted with enormous significance. Take the first line for example:
They say when trouble comes close ranks, and so the white people did. But we were not in their ranks.
There's nothing in those two sentences that a fifth-grader couldn't understand, but they open up a whole world for the reader. With the first line, you know you're not getting a comedy. You're in a time and place when "trouble," whatever it is, is a common occurrence. You're entering a situation that calls for a community to come together, and in the novel's world, that community has to be racially homogenous. You know with the next line that the narrator's family doesn't fit within the category of "white people," and, as the novel goes on, you know that just because they're not "white," doesn't mean that they're black, either – they're Creoles. Throughout the novel, everyday language is masterfully engineered to describe extraordinary situations.