If there's a "great American novel," The Adventures of Huckleberry Finn is it. No other work better embodies American life and attitudes. Huck and Jim's adventures aren't those of a boy and an escaped slave any more than Ulysses' Odyssey was about a king trying to get home—they're the adventures of every American struggling for identity, moral purpose, and a sense of geographic stability. Most of all, they're the adventures of a population growing out of the carelessness of adolescence to the more mature business of establishing a national identity.
To be offended by Twain's liberal use of the "N-word" is to miss the point. The language of his characters is as ignorant as the people who spoke it, and reflects no deeper commentary. Huckleberry Finn is too huge to be reducible to one theme (like racism or slavery), just as America isn't "about" one thing. The novel is purposefully huge and unrestricted, like the nation Twain observed.
In fact, it's the language which distinguishes the novel. Twain's mastery of Southern dialect is unrivalled; that he can maintain Huck's uneducated voice and consistently bad grammar for the duration is testament to his genius, and what draws us in. The story is intriguing enough with its faked deaths, family feuds, bad Shakespeare, and endlessly changing identities (not to mention Jim's dangerous bid for freedom), but one senses that even if the plot was boring we'd still listen to Huck talk. His voice is so thoroughly his voice.
But the significance of Twain's style is more far-reaching. He gives voice not just to an ignorant Southern boy, but to an entire nation (no small feat considering the still-fresh regional animosity of the Civil War). Huck is intelligent and ignorant, experienced and innocent, cunning and naive. His exploits are often boyish daring, but just as often they're serious expressions of his developing morality. They echo America's youthful bravado and rapid growth to maturity by virtue of necessity. Thrust on the world stage, America was forced to behave itself accordingly, even while displaying a distinct antipathy toward taking itself seriously.
Ultimately it's Twain's absurd, brilliant sense of humor which makes us pay attention. He consistently winds us to dizzying heights of hilarity, only to drop us in the middle of some tragic situation, as when Huck finds himself in the middle of a family feud of comically epic proportions that suddenly reveals its darkest underside when he finds his new friend dead after an attack from the rival clan. But there are also altogether lighthearted moments, such as the King and the Duke's "recitations" of Hamlet to a confounded audience of backwoods yokels.
Huckleberry Finn is one of the few nearly-perfect novels. Twain's eloquence, humor, and incisive intellect form a complete literary achievement that makes most serious novels seem like picture books. It's wildly funny and inescapably sad, rational and emotional, universal and specific, vice-ridden and virtuous. It is, in short, the most honest tribute to the American nation that has ever been written, or likely will be.