This is a really good question. I must first say that Cormac McCarthy's The Road is probably my favourite novel. That being said John Hillcoat does about as good as he could do. How does one turn McCarthy's poetic prose into film? Besides the expanded mother flashback (apparently Charlize Theron really wanted in on this movie) I recall a few things that were different. As sparse as the action scenes were, they seemed longer in the movie. I also notice a small bird fly off when they got to the ocean which I don't recall in the novel. Any signs of life were something good; animals like birds should be regarded with a sense of awe. Still the book is largely a meditative, surreal and heart wrenching experience. This is very difficult to translate to film. I'm going to add an article that is pretty good on this subject. It will be in the source link below.