Disregard the superb narrative techniques. Disregard the excellent plotting and characterization—particularly between a mother and a daughter—that throttles the both of them to a climactic showdown where, literally, the fallout of their relationship determines the fate of the world. Disregard the fact this is the first time an author has won three Hugos in a row and disregard all of the other milestones of this series.
What most came through Jemisin's masterwork, for me, is this...
How people use people. As impressed as I was by the structure and texture and other aspects of her novels, I found myself mostly grasping her story as a metaphor for our own—a story of our society; a history.
As Andrew Liptak said, in his article for The Verge, Jemisin is really talking about "...the dangers of marginalization, enslavement, [and] oppression..." and that message about our own history is what has come through to me most forcefully.
Examples of how people use people aren't just sprinkled throughout the book, they define the book and the world which Essun, Nassun, the stone-eaters and all of the characters must struggle in.
For instance, (1) the stone-eaters were initially a select group of individuals genengineered for a specific purpose (to open the Obelisk Gate and extract so-called illimitable power) and those abilities were harvested (genetically?) from another people, the Niess, who were wiped out by genocide.
Furthermore, from the first book, (2) there's the existence of use-names for every person, young and old. In a comm, everyone has a use or they don't belong. These use-names, for example, extend from knapper to doctor to leader.
And, at the heart of Essun's struggle—the main character's struggle—is her past as a Fulcrum-trained orogene, exemplified by the (3) antagonistic Guardians (who are sort of like the will of Father Earth possessing humans; Father Earth using humans because humans once attempted to extract the power of the Earth for themselves).
Essun must pick apart the aspects of her character which are a direct result of (4) oppression— orogenes are used by the Fulcrum to quell the malicious quakes and eruptions of Father Earth; of (5) marginalization— normal people, or stills, despise and distrust orogenes to murderous proportions; and of (6) enslavement— particularly powerful orogene children are yoked and chained into a chair, drugged into a coma, and their powerful, earth-bending innateness thus controlled directly by the Fulcrum.
And these are just a few examples of the intricate relationships Jemisin has created between states and people, subgroups of people, castes and social organization, that all sum up to how people use people in a myriad of subtle and not-so-subtle ways. Affecting all of the personalities and characters throughout her world.
Jemisin has a created a world not so unlike our own: where a society survives only because some people (must?) suffer. A class system of sorts. Rife with examples of all the motley ways—manners, speech, body language, laws—that oppression persists in human society and shapes our own personalities, decisions, choices, lives.
There are many other elements to this story—like the relationship of mother and daughter, parent to child. But what stuck out most to me, the residue I keep carrying with me when I think of this magnificent story, is how we—as a modern people—still have not figured out a way to stop thinking of ourselves as tools.