I have always struggled with openings, and you can't beat Scalzi's first paragraph:
I did two things on my seventy-fifth birthday. I visited my wife's grave. Then I joined the army.
There's nothing subtle about the title. The army of the Colonial Defense Forces is composed of old men and women recruited from Earth before they kick the bucket. Needless to say, they don't fight in old men's bodies, rather their minds are uploaded and transferred into green-skinned killing machines that vaguely resemble the original host at 25, but (much) buffer and better looking. It's a fantasy that appeals to anyone past thirty, and for the first week after the CDF soldiers' new bodies are assigned they get a short leave that allows them to enjoy the bodies any way they want, which mostly means screwing each other's cybernetically enhanced brains out.
This was one of my issues with the book. I really wanted Scalzi to explore more deeply what it meant for an old person to be given a young person's body, and I'm not convinced he did. In general I don't see the old soldiers acting with the restraint and wisdom you'd expect 75 years to give them. Sure, maybe some would go crazy with their new package, but others, such as the narrator John Perry, might be restrained by what they'd lost in the past. And I'm not sure they'd so quickly allow themselves to be assimilated into a bunch of obedient little privates.
In any case, once the fun's over, they quickly become alien cannon fodder, with three quarters of them getting killed in their two to ten year enlistment. Publisher's Weekly in their review compares Scalzi's work to that of Robert Heinlein, and they are not talking about Job: A Comedy of Justice. Once the fighting starts this book is hard-core milSF, which I admit is just not my cup of tea. Cory Doctorow narrows it down and calls the book Starship Troopers without the lectures, but OMW makes the point nevertheless, which is that life without mass organized cooperative killing is just a pale shadow of itself.
MilSF has its own tropes, and this book covers them. One of the tropes of MilSF in specific and military fiction in general is that anyone who advocates for peace, especially someone who hasn't earned their stars in battle first, is a self-important idiot who will be humiliatingly massacred, proving the point that the stupid naive civs just don't get that The Bad Guys Are Out To Kill Us. Sometimes this is really true, and after all in this case we're talking about imaginary alien civilizations, just as in ST. But often in real life this attitude, often adopted by people who themselves have no particular military experience, can lead to stupid military overzealousness, to which I won't even bother to link. It's the chickenhawk argument bizarrely turned on its head: you aren't entitled to make peace unless you've already made war, meaning any long-lasting peace is by definition impossible. Scalzi might argue he intended to make no such point, but the author's intentions become irrelevant once the 101st Fighting Keyboarders get their hands on his words and start using them for their own purposes.
Another significant MilSF trope is that an important turning point in the plot always comes down to some one-on-one duel of honor between a human and an alien or bad guy (or five-on-five, in this case). This is usually because of some eccentric cultural custom of the alien bad guys, by which in spite of their overwhelming military superiority and unimaginable ferocity, they're suddenly willing to cede huge advantages because one human kicks one alien's ass. I have never been in the military, and never will be unless the CDF starts recruiting around 2045, but I know enough to know that war just does not work this way anymore, if it ever did (which I doubt). Don't get me wrong, I understand why this is necessary plot-wise, because actual mass technological combat is just far too messy and imprecise to be described with enough narrative precision to be the climax of a book. But this just happens to be a bridge too far of suspension of disbelief for me personally, maybe because of my wimpy naive civilian pacifistic tendencies, which would incline me to get devoured bloodily by the first genuinely ferocious alien that stumbled across me.
Don't get me wrong, I greatly enjoyed the book. The action was fast-paced and engaging, the main character likeable, and there were some interesting meditations on what it means to lose a person and our need to replace them. And I'd add that the book is far deeper than most MilSF I have seen out there. It's just that now that I know what Scalzi's fiction is about, there are a lot of other authors that are higher on my priority list before I'm ready for another dose. Anyways, there's always his ever-popular blog, which I'll continue to read daily.