In the immortal words of Roman Strauss, “This is all far from over…”, but I think I’ve read enough of the Old Man’s War universe to say a couple of things. Firstly, Scalzi is a perfectly good writer; you zip through the prose painlessly, and there’s enough good ideas getting chucked around that only a perfectionist would point out that all the characters seem to source their dialogue from the same shop. Scalzi is entertaining as all hell, and would be great fun to have a pint with, but a wry tone which works in real life isn’t a cure-all for every voice in a book.
From what little I’ve looked at in the inter webs, most of the chat about Old Man’s War is about how just what kind of commentary it forms on vintage Robert Heinlein. Some are delighted that Heinlein’s world view is being challenged and updated, and some are spitting blood around the place that the sturdy noble ideals of American yesterday are being defiled, or more bafflingly, are delighted that Heinlein’s charmingly outdated notions of martial valour are being refreshed and promoted for the nihilistic youth of today. And it’s lovely that everyone thinks this matters; it’s sweet that people are prepared to pull apart a bubble-gum fiction and tan its hide into a coat which drapes fetchingly over the flabby limbs of their set opinions, whatever they might be. I’m sure even Scalzi has an opinion on it, but I haven’t gone to look it up because if I can’t figure out what he’s trying to say from what he’s already published, he’s doing something wrong.
But never mind the sociology; let’s just unpick the big begged question in the middle of the whole thing. Ground combat in an era of stellar colonisation is bullshit. Whatever his reasons for doing it, Scalzi is telling a story about a universe in which squabbling alien races invade planets and wipe out the locals so that they can take the planet over lock stock and barrel. They don’t conquer the planet to enslave the locals and profit from the stuff which is there. They don’t want slaves, but lebensraum, and the local produce doesn’t meet their needs (though for some of them, the locals at least represent a tasty snack).
Well. If all you want to do is wipe the population off the face of a planet and you don’t really care about property damage, there’s no real need to get mud on your boots. If you’ve managed to mobilise the energy to move colonists en masse from one star to another, you’ve got all the energy you need to wipe out any civilisation from orbit without ever getting out of your chair. Stuff and nonsense, you splutter. The targets will be protected by their own space fleets. No doubt you’re right. But again, that’s a space fleet. It will be in space. You’ll have to subdue it if you want to get anything done to the planet, but that’s not going to involve the infantry. And once you’ve done that subduing, you’ve got local space to yourself and there’s the target at the bottom of a nice deep gravity well, defenceless. There is a reason why there isn’t a figure of speech “training fish to hunt other fish in a barrel”. Unless you want to be a dick about it, the cliche is a lot less effort (though as Mythbusters have demonstrated, you need to choose just the right amount of gun if you want to keep the barrel for later). Since you’ve got access to the kind of energy which lets you move colony ships from star to star, you’ve got more than enough energy to move rocks around the system and chuck ‘em at the scurrying savages below. The only reason to use infantry after that is winkling out the stragglers, though even that task is probably best left to collateral damage from the massive biological weapons fiesta you’re going to need to unleash to refashion the planetary surface to best meet the dietary needs of your colonists.
This energy question is the most usual counter to the Drake Equation and the Fermi Paradox, which can be boiled down to “If there was intelligent life in the universe, surely we’d have seen it by now?” . Nobody, the argument goes, has dropped by because the laws of physics mean that it’s impossible for anyone to move a significant mass out of a planetary system. Conceptually you could move quite a lot of mass very slowly out of a planetary system, but even doing it slowly consumes so much energy and other resources that you’d get more of a return on it by building something locally. Thus, no-one’s coming by any time soon, because what’s the point for them? There’s nothing you could conceivably get from interstellar travel that you couldn’t get for less effort by making it locally.
As far as we know. You can always rely on people to remind you that no-one thought you could get a man to the moon, or safely travel in a train going faster than 20 miles an hour, or have a pocket sized computer which can report on your precise whereabouts to the NSA every thirty seconds while telling them everything you’re saying - and all at your expense. Yes, the state of our scientific knowledge keeps changing, and what seemed impossible to us fifty years ago seems trivial now. But if there is some cosmic hyperdrive out there which makes interstellar conquest an affordable (even for Halliburton style affordability) idea, you’d think that by now someone would have tripped over it and started taking joyrides. Which suggests - until the little green men show up, at least - that our current understanding of the universe’s speed limit is about right, and this here solar system is all we’ve got.
Scalzi knows all this; at some level, anyone educated enough to enjoy his fiction knows it too. We just suspend our awareness of the impossibility of it all so that we can enjoy the actual story. Stories of travel between the stars are fun, all the more so when they echo stories we’ve told for centuries about travel between different lands here on earth. Most SF about space travel is patterned firmly on stories about 19th century sailing ships. It’s dangerous, chancy, cramped and highly technical, and at the end of the voyage there’s people who are strange and yet just like us, and you buy their stuff or hit them on the head and take their stuff - or make them into stuff, and take that stuff home for fun and profit.
For far future fiction, it’s surprisingly backward looking, and that’s probably a big chunk of its appeal. The future is frightening, and it’s reassuring to see that it will be just like the past. The past might have been horrible, but by definition we’ve survived it, so we’ll survive the future if it’s just more of the same. And high adventure’s always a fun read; the bold hero out thinking the faceless others and bounding to an unexpected yet inevitable victory.
There’s a metric buttload of that going on in Old Man’s War, with the humans forever out-thinking and out-punching the aliens despite starting out hopelessly behind them in the space race and just about everything else. It’s jingo as all get out, as is the assumption that in the future all heavy lifting will be done by the United States or people who would fit right in there. Which is where the sociology I was talking about earlier comes from. Is Scalzi just writing SF about an idea which interests him and writing from what he knows rather than making a hash of trying to write a more cosmopolitan viewpoint? (heaven knows that this would be a smart enough move for him given his voice issues). Or is he playing a long game about American exceptionalism by showing us first how it feels from the inside and gradually hollowing that out until it collapses from its own contradictions? You can certainly make a case for that, since each book has chipped away more and more at the simpleminded Heinleinian certainties of the first book in the series. How much of that was the plan from the outset and how much a smart writer course correcting from feedback, I really can’t say. The only thing I can say for sure is that Scalzi knows enough to know that his books aren’t about the stars; one way or another, they’re about the wars we’ve had, or are having, or will have, in our very own backyards.
 Related SF problem; time travel has to be impossible, since if it was possible, it would have become available in the future and someone would by now have come back to tell us about it.
Charlie Stross has made a similar point one scale down by contrasting the idea of colonies on Mars with the current reality that we can’t figure out a way to sustain human habitation in the Gobi Desert, which is way more hospitable to human life than Mars, and a much shorter commute. (If you follow the link, you’ll also get a very long, very detailed version of the argument I’m trying to put in a single paragraph above)