For me, it is the relationship of the father and the son and how this is presented throughout the novel. The father is shown to be a desperate man, trying to do whatever it takes to secure his son's safety and protection. However, the son seems to act as some kind of latent conscience of humanity. He at various stages of the novel prevents his father from losing the lingering traces of humanity that remain. For example he makes the father give the old man they come across food and stops him from being cruel to the robber that steals their belongings. This makes their relationship very interesting and adds to the excellence of this book.
It was a tough book for me to get through, there are so many stark and depressing mental images and events, but I am a big fan of Cormac McCarthy's ability to take that which people do not want to think about and portray it in terms which are still beautiful in some way.
His command of the language, or more appropriately, the arrangement of the language, is both unique and artful, not to mention a critical element to the effectiveness of his storytelling.
This is a discussion question, but I'll start it.
The Road is beautifully crafted. McCarthy's ability to transcend the post-apocalyptic genre with lean, tender poetry is a small miracle. Just look at this passage:
The road crossed a dried slough where pipes of ice stood out of the frozen mud like formations in a cave. The remains of an old fire by the side of the road. Beyond that a long concrete causeway. A dead swamp. Dead trees standing out of the gray water trailing gray and relic hagmoss. The silky spills of ash against the curbing. He stood leaning on the gritty concrete rail. Perhaps in the world's destruction it would be possible at last to see how it was made. Oceans, mountains. The ponderous counterspectacle of things ceasing to be. The sweeping waste, hydroptic and coldly secular. The silence.
McCarthy's weave of imagery, description, fragments, and simple sentences reads like a poem. It should be depressing, but it's not. His prose-verse is so full of life and passion that it overshadows the bleakness of its subject matter.
The dialogue between the father and boy is even better. In such a violent and gloomy world, the pilgrimage of father and son is a testament to McCarthy's own love for his young son. Their back-and-forth question-and-answer dialogue, devoid of quotations or pretense, is simple and elegant and provoking. It's a traveling Socratic seminar about the meaning of death, nature, God, and humanity.
As a father of a young son, I love the book, above all, because it is a love story of a father and son's sacrifice for each other.