Hussar said:
See, to me, the problem with CSL's idea is that it is too static. There are just far too many things in D&D that can disturb that social order. Things that are pretty easily done as well.
Easy travel - flocks of hippogriffs. All it takes is one nation to start breeding hippogriffs and suddenly they become as ubiquitous as horses because they are too much of an advantage NOT to do. A small flock of 150 hippogriffs could absolutely change the face of conflict. Not to mention commerce.
For a look at what a D&D world should probably look like, I usually point to Terry Pratchett's Discworld.
I disagree on the hippogriff part. First of all, where are you going to find enough hippogriffs for them to breed? How many mistakes are you going to have to make-killing much of the available hippogriff population in the process? You'd essentially have to learn horse-breeding all over again, making all the very same mistakes that the earliest breeders made.
And besides, are hippogriffs really trainable in war? Do they panic too easily, or are they fast enough in combat? I can see armies using hippogriffs and griffons as scouts, but they'd be a pain in the you-know-where to keep fed and stabled, and it'd probably be too expensive to do over the long run as comapred to horses.
Then there's commerce-horses only have to pull a wagon of goods along, with gravity keeping it attached to the ground. Hippogriffs or other flying creatures would have to carry it below them while flying, with gravity pulling against it. Just how much of a weight would that be? How many hippogriffs would you need? How much would it cost to keep them all stabled and fed? Hippogriffs aren't that strong-they lose a step when carrying one passenger and his gear. Could they really be useful in commerce when a team of horses can do so much more? Sounds too expensive to be worth it.
Since, in my version, man will never invent the internal combustion engine, he will never be able to fly without magic. Also, since in my version you have to be 18th level to manufacture a permanent magical item, and characters of that level are as rare as a twelve-leaf clover, most merchants will simply accept the risks of travelling overland or by sea, since flying is simply impossible.
My attempt here is to show how, by altering some of the mechanics or parameters of the setting, you can easily make your campaign much more medieval, if you want. It's a simple matter of DM fiat. Maybe the RAW are more 21st century, but who says they have to be that way?
There's a reason part of it was called the Dark Ages, after all, and not everyone wants to play a plague-bearing hobo with no coin and an early demise on the horizon. 
