The man is famous for his speeches, including the power and directness of his vocal style, but as a writer, he was technically as perfect as anyone. This manifests itself in two ways: organization and style. His sermons balance emotion and logic in such a way that his words resonate powerfully for believer and non-believer, Christian and non-Christian, advocates for justice and those still undecided. In "Loving Your Enemies," he says, "Do I sound like most preachers--idealistic and impractical?" No, he doesn't--his arguments are grounded in common sense and history, and the organization of his writings are organized systematically, following near-traditional five paragraph structure in order to make his argument as clear to understand as his points are timeless. For example, in "A Tough Mind and a Tender Heart," he defines tough-mindedness and then tender-heartedness before synthesizing the two, and in "Letter from Birmingham Jail" he systematically refutes his naysayers before building to the point that "one has a moral responsibility to disobey unjust laws." In addition to his organization, his arguments are near air-tight in their support through the use of evidence both ecclesiastical (from the Bible) and secular, including Aristotle, Hegel, Emerson, and Lincoln. His ideas, such as suffering for justice and committing to nonviolence, are so fundamental and powerful, and that fundamental power is echoed in the structure of his argumentation.
The second point is that, stylistically, he was a master of the language as well, with the technical power and poetic fluidity of his prose comparable to the likes of Melville and Woolf. Specifically, and most interesting to me, are his use of fancy-sounding rhetorical devices, among them antimetabole, isocolon, and anaphora (which I'll define in a second), that imbue the force of his arguments with a euphony that catches your ear and stays in your head like a song with a great hook. Just a few examples of each:
Antimetabole - juxtaposing a transposing of two words, phrases, or clauses (a/b/b/a)
"The militant are not known to be passive, nor the passive to be militant."
(from "A Tough Mind and a Tender Heart")
"Hate destroys a man's sense of values and his objectivity. It causes him to describe the beautiful as ugly and the ugly as beautiful, and to confuse the true with the false and the false with the true." (found online in multiple places, source unknown)
Isocolon: repeating parallel grammatical constructions
"The Bible, always clear in stressing both attributes of God, expresses his toughmindedness in his justice and wrath and his tenderheartedness in his love and grace." (from "A Tough Mind and a Tender Heart")
"God is neither hardhearted nor softminded. He is toughminded enough to transcend the world; he is tenderhearted enough to live in it."
(from "A Tough Mind and a Tender Heart")
"Every true Christian is a citizen of two worlds, the world of time and the world of eternity." (from "Transformed Nonconformist")
"His goodness was not found in a passive commitment to a particular creed, but in his active participation in a life-saving deed; not in a moral pilgrimage that reached its destination point, but in the love ethic by which he journeyed life's highway." (from "On Being a Good Neighbor")
Anaphora: Repeating an introductory word or phrase
"Let us march on poverty until no American parent has to skip a meal so that their children may eat. March on poverty until no starved man walks the streets of our cities and towns in search of jobs that do not exist. Let us march on poverty until wrinkled stomachs in Mississippi are filled, and the idle industries of Appalachia are realized and revitalized, and broken lives in sweltering ghettos are mended and remolded." (from "Our God is Marching On!")
"Will we continue to march to the drumbeat of conformity and respectability, or will we, listening to the beat of a more distant drum, move to its echoing sounds? Will we march only to the music of time, or will we, risking criticism and abuse, march to the soulsaving music of eternity?" (from "Transformed Nonconformist")
Many preachers and orators have mastered these rhetorical techniques, but few have paired them with the power and clarity of Dr. King's arguments.
But perhaps the most incredible thing about Dr. King, to me, is an aspect of writing that is not on the page, and that is his ethos. This is a man who faced countless death threats, privacy invasions, vitriolic criticisms, and unjust imprisonment on top of the racial prejudice experienced by a black man growing up in Georgia. Consciously or unconsciously, he was well aware the animus against him. There's a very real possibility he knew his fate. And still he pressed on, he lead people with faith and fortitude, he resolved to never retaliate with violence, he determined to show love and compassion to those who opposed him. The man stuck to his (lack of) guns. He embodied his words through his actions, and through those actions, his impact on our ability as a culture to have understanding and compassion for people so unlike us--no matter their race, religion, ability, philosophy, personality, appearance, etc.--is deserving of canonization. It is an ideal worth striving for.