My mom's comment about the book was that it seemed to be almost a list of how to kill people in new and interesting ways. The book does have a lot of killing in it, of course, since it's about war, but once I pressed my mom a little she admitted that it also was about what makes a person a hero. You spend the book learning about, and sometimes getting attached to, a small number of characters, but which of them is the real hero?
And the book is also, as my husband pointed out, an anti-war novel. Although many of the characters are concerned with glory, being brave and making names for themselves, much of the story is written in a way that shows what a waste war is. Young men's lives are cut short before they have an opportunity to be what they could have, or should have, been in life, and often over quarrels that had nothing to do with the men fighting and dying for them.
And of course, the book is excellent fantasy. According to my husband, who has read the author's other books, The Heroes is connected to the others -- it takes place in the same world, and some of the characters are mentioned in other books -- but it still stands very well on its own.