In fantasy worlds there is a always a strange mishmash of feudal culture and feats of magic and heroism. Sometimes it works and makes sense and other times ... not so much. There are plenty of troubles with the classic DnD world organization but most of them tend to revolve around just how big a shift high level adventurers must cause in culture, warfare and politics.
It is generally relatively easy for a high level party to meet up with a 10,000 person army and utterly demolish it. The wizard ends up flying above the massed enemy while invisible and massacres them by throwing enormous fireballs around willy nilly. We should keep in mind that historically very few armies actually fought until they were destroyed; by and large a small percentage of the people involved would die and one side would break and run. I can't help but think that it would take only a few random explosions in the ranks to completely break an army, especially when they have no idea of the source. Not only that, but how would you protect a ruler? The king or queen is likely to be a normal person with 10 HP (or less!) so at any time a group could teleport in, massacre the ruler and teleport or fly back out at will. That of course assumes that they *need* to leave and don't just kill everyone in the castle and raze it to the ground, which they usually could.
So how can we construct a society that has incredibly powerful people (I will call them Heroes) in it that makes sense? One way is to simply restrict the number of Heroes around. Building a world where only the PCs are Heroes is possible but it does mean that the PCs can overthrow nations at will and cannot face any Hero opponents which I am not sure is a good idea. This strategy also entails that the PCs be unique and epic which does fit a lot of storylines but certainly not all. It also means that once the main storyline is complete your world is probably unusable since the PCs presumably can rearrange the entire world to suit their liking. I like a world that is a little more durable than that and which doesn't rely on the PCs being so very unique.
Regularly the way DnD worlds are constructed to avoid this is assigning hereditary monarchs tons of bonus HP or character levels for no reason or simply layering magical protection over the ruling family and/or class. This doesn't solve the problem of army combat not making any sense though and it requires all kinds of additional spells and effects to achieve that aren't in the books since the baseline rules don't allow for protecting people sufficiently to avoid teleport bombs.
The idea I am leaning towards for my next world is one that rewrites military conflict very substantially, making it much more like modern conflict than medieval battles. In modern society you have security guards, police and other enforcers who keep people in line in times of peace but when a tank or bomber shows up the security guards don't get together in a big group to fight them, rather they wait for the army to do something about it. I imagine a fantasy world the same way where regular guards and soldiers who are not Heroes are fine and well for guarding shops, fighting bandits and other such tasks but when real war comes the kingdom calls on its Heroes to do the actual battle since they are the tanks and bombers of their world. These Heroes would also end up being the rulers of their world since hereditary monarchies of normal people would be so fragile. I see groups of Heroes ruling nations with structures varying from benevolent ruling councils to iron fisted dictatorships where the most powerful and ruthless Hero is a terrible despot. This might even have a strong mitigating effect on warfare since if you have to go out and actually fight the enemies yourself and risk being fireballed to death there is a real incentive to not declare war on another nation.
4th edition DnD actually has many less problems this way than 3rd edition. Wizards can't just fly around invisible blasting people because everyone's combat abilities are strongly restricted and things like teleportation are much less convenient. A 4th edition DnD party can beat up a lot of bad guys but they simply can't fight 10,000 guys with bows; they get turned into very dead pincushions on round 1. For this I have to give 4th a lot of credit, you don't need to make up your own political structures and military designs from scratch and can just modify historical ones slightly to fit. There are really powerful and dangerous people around but normal people can be leaders and survive and armies make sense. Somehow though I desperately want to play in a 3rd edition / Pathfinder world; perhaps it is only nostalgia but it is there nonetheless.