The language in The Things They Carried is not difficult to read. The vocabulary isn't simple, but it is modern, and the writing flows beautifully. The characters' dialogue carries the reader along easily and is a lot of fun to read, and it's easy to figure out a lot of the soldier-speak from context clues.
The readability of this book what keeps it from being Everest-level, folks, because while the words may be easy to read, the ideas contained within the book are not easy to understand. You should absolutely not feel silly for feeling confused when O'Brien jerks you around about what is truth and what is not, because everybody feels that way. This is a big, complicated postmodern book about a big, complicated war. Fortunately, O'Brien doesn't use words like "postmodern" in his book. He takes away the fancy college vocabulary and strips the ideas down to their bare bones with words like "story-truth" and "happening-truth," which makes the book accessible even as it is difficult. Very interesting work, Mr. O'Brien.