Moby-Dick is not The Great American Novel—it is simply The Great Novel. It's themes are too universal, too utterly human to be restricted to those of a single nation or continent. It's not even very clear Melville was just writing a novel: his masterwork is a hymn, a poem, a philosophical treatise, a celebration of the sea, and a mystical adventure of the soul as terrifying and powerful and unknowable as the White Whale himself.
From the celebrated opening line ("Call me Ishmael," a three-word sentence better than entire books by lesser writers) to the haunting last chapter, we are gradually drawn away from everything we know about ourselves and the world, and into a beautiful wilderness of words, images and ideas that are as ancient as they are modern. The hunt for Moby-Dick perpetrated by Capt. Ahab and his subservient crew is not just a common whaling voyage or even just an uncommon revenge tale. What Ahab wants to subdue is not an enigmatic whale—it's the secret and meaning of life, death, evil and God, lost at sea just like all of humanity.
Melville was a chronic innovator, manipulating language the way Ahab manipulates his crew, getting it to do whatever he wanted. Ishmael ostensibly narrates, but at times his voice seems to fade into another and whether it's Melville's or some other that we're hearing is anyone's guess. Information he couldn't possibly be privy to is described by Ishmael in detail, including events which transpire in Ahab's cabin, a sort of demonic holy of holies from which he emanates to direct the Pequod like a pagan god visiting terrified congregants.
Some critics complain there's too much detail. This is only true if you're looking for a simple plot, characters, convention, etc. What Melville offers instead is an evocation, an exploration, a quasi-religious experience. That's not to say there isn't a plot or that the book is directionless—there is, and it's not. In fact, there's more excitement in these pages than in a lot of what passes for adventure stories, scenes of whale hunts, attempted mutiny, storms, the mystery of St. Elmo's fire, Ahab's charismatic speeches. But none of these elements (or the descriptions of whaling tools, species of whales, etc.) exist in the narrative for their own sake; each is symbolic of something else, something at once more fundamental and more universal than the representation.
The White Whale himself, of course, is the central symbolic figure, his true identity still as much a mystery to readers and critics as Melville's limited and baffled readership found him on the book's initial publication. Their trouble is they want him to mean something specific, to have one purpose in the story, when in fact Moby-Dick represents many things, some of them contradictory. He represents Ahab's madness, the crew's blind obedience, the destructive force of the ocean. But his identity goes much deeper: Moby-Dick is human sin, man's obsession with evil, the soul of innocence and simultaneously of guilt and death. Moby-Dick is essentially human nature embodied, and Ahab's relentless pursuit of him is the struggle for self-identity, the knowledge of good and evil, even the knowledge of God.
Delineating the many facets of the Whale's symbolism in a short review is about as pointless (and impossible) as Ahab's insane quest. This is not a typical novel with "good guys" and "bad guys" and a central conflict. This is Melville's attempt at a cosmology, a philosophical investigation that is also an aesthetic masterpiece, and understanding it is as spiritual and intuitive as Ishmael's bond with Queequeg. When people reference "great literature," this is the kind they're talking about. A book to take your breath away, Moby-Dick is one of the great works of human imagination.