Editing The Imperium of Man (section)

===In Summation===
To put it simply, the Imperium is more or less a Feudal society rather than an "Empire" in the modern sense like is seen in Star Wars or a thousand other Science Fiction/Space Fantasy stories. You have a central authority in the Adeptus Terra and a unifying religion in the Imperial Creed, but the assorted fiefdoms of the Imperium generally are left to rule themselves without that much oversight from the central authority as long as the taxes come in on time and no open rebellion is occurring. Given that the Imperium is 40k's counterpart to the Empire in Warhammer Fantasy which is more or less the medieval/renaissance era Holy Roman Empire but bigger and more technologically advanced, this isn't all that surprising. And much like those feudal era societies you have some parts which are much better off than others, in agriculturally rich land with ample access to trade and developed cities feudal societies could rival anything that the earlier societies of antiquity could make in terms of sophistication. But poor provinces mostly made out of useless swampland and rocks are of course going to be poor hicktowns (somehow their ancestors thought settling in said swamp and rocks was a brilliant idea). Furthering the parallels to feudal Europe is that the Imperium is built atop of the corpse of an older and more urbane society; Rome for Medieval Europe and Dark Age humanity for the Imperium, and emerged out of a prolonged dark age following the collapse of that old empire. And the most important thing to note is that Feudal governments did not exercise absolute control, for a feudal peasant your local Baron had far more influence over your life than the King or Emperor would, and how harsh or kind your life was depended largely on the nature of your local overlord. Similarly, some places would have elected heads of local government like mayors in many cities, some would be ruled directly by the church, and even the laws and customs could vary dramatically between the domains of the various nobles. In essence, the Imperium is a space medieval society that due to the original generation of games workshop writers being history nerds with degrees and shit, actually resembles a medieval society much better than stories supposedly set in faux-medieval times. The Imperium has much more in common with the Empire of Charlemagne (during the Great Crusade) or the Holy Roman Empire than it does with any of the expansionist and/or authoritarian regimes that arose from the 1700s and onwards.

You could also hand the whiners the Ciaphas Cain books, which repeatedly showcases Imperial worlds that are actually fairly decent places to live, with planetary governments that actually give a shit about their people, and are perfectly capable of surviving an invasion or two (or several, in the case of ork threats) and still returning to a state of normalcy after the organizations whose purpose is to deal with these threats does their jobs (who, by the way, eight times out of ten are usually able to do so without leaving the planet in question a smoking ruin).

Space feudalism is even implied in the way the people in the lore speak of the Imperium. For instance, a politician might tell his fellow "watch what you say or the Imperium will come" or something. The Imperium is an outsider to most of the worlds it is made of, not an immediate presence. This is, like real life, out of necessity. Real life feudalism was due to social breakdown following Rome's collapse, and technology limited just how much one group could control. The Imperium is much the same, this time due to sheer scale involved and the collapse of order following the Iron War and the Age of Strife.