I suppose one of the most obvious things we can point to that is left out in this excellent account of the Vietnam War is the way in which the author only really gives us background information about Lieutenant Jimmy Cross and his life before Vietnam. We are given passing comment on other characters such as Kiowa, but one thing that is definitely not told to us is the kind of lives that they had before coming to Vietnam and how these backgrounds result in the kind of person that they are in the story.
We can see this through the massive importance of Martha's letters to Jimmy Cross. They are clearly a fundamental part of his character and motivation, and his burning of them at the end of the story because of the way in which he perceives them to have distracted him from carrying out his duty is incredibly significant. We receive hints of similar backgrounds and objects for other characters, for example Kiowa's possession of "his grandmother's distrust of the white man, his grandfather's old hunting hatchet." However, such objects are left undeveloped and you have the sense that there are stories to tell about these objects just as there is a story to tell about Jimmy's letters from Martha.