Facinating read.
I'm not 100% convinced that the Middle Ages were quite as stagnant as SHARK claims. I have read (and I am certainly no scholar, so, feel free to beat me over the head for having my facts wrong) that labour during the Roman Empire period was something like 90% done by humans. By the 12th or 13th century, in Europe, this number was down around 30%. This, to me, certainly points to some advancement.
I really wish I could remember the name of that damn book I read - Waterwheels and something or other. Arches and Waterwheels? Damn I cannot remember. Can someone point me back to it? Excellent read anyway.
But, putting aside the historical end of things and relating this back to fantasy RPG's, there are a couple of things that pretty much remove D&D from any sort of historical context. The big one, in my opinion, is the existence of alignment as a force in the universe.
Think how much it would change the world if you could definitively point to something as really good or really evil. No chance of failure. This would have massive impact on the development of a society. I could easily see groups of paladins (3e or earlier) running around, detecting evil and slapping a Helm of Opposite Alignment on anyone who glows.