What were the things that the soldiers carried—not weapons and such?
The author (O'Brien) talks about things the soldiers carried that were not war-related in terms of battle, but things that held a personal significance or tied them in some way to their habits, etc., of life at home.
For example, one man carried letters from a girl friend (they were not romantic letters, but Lt. Cross had hopes that one day they would be). One man carried a toothbrush and dental floss, and another soldier carried tranquilizers. Others carried a diary, comic books and a New Testament. Some men carried insecticide, suntan lotion, Vietnamese-English dictionaries, chess sets, and basketballs. Many of the men carried photographs of people at home.
Above all, the men carried heartache and worry, love for their "people" and hate for the war and all that came with it.
For the most part they carried themselves with poise, a kind of dignity. Now and then, however, there were times of panic, when they squealed or wanted to squeal but couldn't...Afterward, when the firing ended, they would blink and peek up. They would touch their bodies, feeling shame, then quickly hiding it. They would force themselves to stand.
In "The Things They Carried," O'Brien lists the many military-issued items that all the soldiers carried, and he lists them: the items sometimes vary depending upon the rank or function of the soldier carrying them. In the midst of these lists, however, O'Brien also lists the items that keep the men tied to home, grounded. It is a reminder to the reader, that although these men are soldiers, they are men first, and members of the military service second.
I believe O'Brien does this in order to remind the reader of the reality of war. It is not something that takes place between two faceless opposing factions in a remote part of the world. The battles involve men (and now women) who are flesh and blood, with hopes and dreams of lives and futures much the same as our own. In this, I think he tries to remind us of the enormous sacrifice these individuals make for their country, and to humanize the individuals so they don't end up being simply numbers or statistics.