It struck me that my thought about this story seemed confirmed: I don't think it's really a novel. Specifically, it doesn't appear to have a plot; at least, it doesn't have a conflict. Oh, there are external, purely physical conflicts, between humanity and the Skinnies and then the Bugs. But I don't see much inner conflict associated with them. The book starts out with Juan Rico saying that he always gets the shakes before a drop—but he's been assured that it's not fear, it's just a neurological reaction. Nor is there significant interpersonal conflict; Juan's father Emilio is angry over Juan's enlisting, as the sort of thing respectable people don't do, but their confrontation takes up all of one sentence, and the next time we see Emilio he's changed his mind on his own. So I kind of see why Scribner's turned this down.
And yet, at the same time, I did find myself moved by the time I had finished reading it.
So what kind of fiction is this?
I think that Northrop Frye's literary categories might be helpful. Frye says that there are four kinds of fiction: the novel, the romance, the confession, and the anatomy. As I analyze them (this is a bit different from Frye's explanation), the novel and romance are primarily about concretes—people, places, and things—where the confession and anatomy are primarily about abstractions or themes. In a novel, you have the emotions shown as taking place inside the characters; in a romance, they're often projected out into the fictional world. Well, in the same way, an anatomy projects ideas and themes out into a fictional world, but a confession shows them inside someone's mind.
Starship Troopers is told in first person, and it shows Juan Rico confronting certain ideas—specifically, the ideas of duty, patriotism, and military service—and discovering their meanings. He starts out as a rich kid (the name "Juan Rico" is plainly allegorical) in a high school class arguing with his teacher; over the course of the book he discovers that his teacher was right, and by its end he has fully accepted his role as a soldier. And the whole story is about how he comes to do so. The battles aren't significant in themselves; their point is to show him performing that role. And that's a much more abstract narrative.
I'm not sure where I'm going with this, but I think it helps make better sense of my response to the book.