A little world-building today.
The impending release of Dark Sun has me thinking about worlds in relation to power sources. It's an example of a world where psionics rules, or is dominant over all other power sources. Actually, Dark Sun is a so-so example; while psionics are more common and dominant amongst the people, the rulers of the world are still arcane spellcasters (the defiling sorcerer-kings). But it's an interesting thought experiment: what would a fantasy world look like and change if a single power source dominated the world, or, at the very least, was the dominant power source in the region.
Most classic campaigns tend to be dominated by the martial power source, mostly because they use real world analogues that developed through our history without magic. Martial is the default. Then, sandwiched between the thinly veiled historical realms will be a wizard kingdom, which still seemingly functions like a non-magical kingdom save the magi elite.
Moving beyond this basic assumption, let's instead work with the 4e idea of martial power and not just the idea of no-power dominating. What would a world dominated or ruled by martial power look like?
It strikes me that a heavily martial world would almost be a meritocracy: rulers and the powerful would be those with the skill and talent to achieve greatness. The early Conan stories had elements of this (king by his own hand), the strong took what they wanted and those with the might and will to secure power would claim it and hold it. This adds an equality to power, as anyone can grow strong in a martial world. It's the equalizer.
Given the advantages magic still has over martial in 4e, mostly in range and targeting numbers, I imagine other forms of power would be discouraged or restricted. This might be the outlawing of magic or a requiring of licences. Or it could just be dismissed. Divine characters would be whelps that need to cry to their god to win fights and wizards would be cheaters unable to life a real weapon.
Really, it's a little like High School with the martial power source being the jocks.
Which actually brings-up an interesting idea. Star athletes tend to do well because of existing talent and money: the financially stable families are the ones that can afford the gear and registration, and can take the time off work for practices and away games. Likewise, in a martial realm, the already influential families would be the ones able to afford fighting tutors and masterwork equipment. Even in a meritocracy there's room for nepotism.
More than other power sources, martial characters are tied to weapons. The choice of two-handed weapon and or sword-n-board might define a person, and everyone might be expected to have a weapon of choice. Weapons might be the equivalent of cars, where you are silently judged based on your choice. Longswords might be the standard four-door standby while rapiers might be the over-prices sports car. Fullblades would of course be SUVs.
The disadvantages and weaknesses of a martial dominated world would be having rulers dominated by strength and dexterity, so there would be few intelligent or wise rulers. Generals ruling through their tactical genius would be more rare, except if they were also warlords. Leaders who were not able to lead by example or action would be more rare. I don't envision many large empires, as the diplomacy and delegation needed to manage large empires might escape muscle-bound dictators. And, as seen by Alexander the Great, Charlemagne, and Genghis Khan, empires built around a single powerful figure tend to collapse after their death.
This is old-standby for DMs and world builders who want a different world from reality, the one that is instead dominated and ruled by magic.
This is done a few different ways. There's the Dark Sun way where a bunch of powerful elite mages keep their secrets and art restricted. Then there's the high magic worlds where mages and wizards are common with arcane schools and academies. The archetypal example is Harry Potter but there are others (Earthsea to some extent). Another take on magical dominance is the magic = technology hook that Eberron ran with.
There's alot of fertile ground for an arcane world but there's also alot of clichés. As "magic" is the only other D&D power source frequently used in fiction there tends to be many more existing stories that have mageocracies.
Unlike martial-dominated worlds, I can see arcane worlds developing larger sustainable nations. The leaders are more likely to be intelligent and cunning and there is greater potential for magical tools such as rituals and alchemical items.
I envision an arcane-centric nation or world a little like academia, with knowledgeable and respected scientists fighting tooth-and-nail for credit and new discoveries, and the accepted and established ideas held all but sacred. For all their claims at being open minded, scientists can really cling to their previous assumptions and still easily descend into black-or-white dualistic arguments. Research and esoteric topics might be frequent, and even the common folk might engage in alchemical or magical experimentation. In some ways this is similar to some portrayals of the Unseen University of Discworld fame.
4e adds some variety to arcane with the defender and leader roles. It's interesting to look at the idea of a magical kingdom and think of its elite group of swordmage knights with artificer and bard support. The king might not necessarily be a wizard and might instead be a warlock or sorcerer. A chaos or infernal influenced land would be very different than one ruled by a wizard.
Theocracies are not unknown in fantasy or the real world. Pharohic Egypt was ruled by warrior-priests who had to strike a balance between being the defenders of the nation and the defenders of souls. Later, they even claimed to be gods themselves. Even in modern days, the Vatican city-state is essentially ruled by a divinely chosen messenger.
In D&D theocracies are not unknown, especially since the existence of the god in such worlds is not a myth and rather factual. It's hard to dispute a leader's claim to the Mandate of Heaven when their god can literally manifest and squish heretics. It's surprising more leaders and kings don't adopt religious platforms, as by following the king in life you can be guaranteed an afterlife.
Absolute good and evil in D&D make this kind of nation less satisfying. It's handy to have a nation ruled by the chosen high priest of the god of sunshine and puppies, because you just know every one of their decrees is a good thing. It's a little hard to protest a tax hike from someone bathed in a holy sunbeam. Likewise, it's hard to think of reasons not to attack and invade the nation ruled by the dark priest of naughtiness, weeds, and paper-cuts. They're not going to be a good person with the welfare of their state on their mind. It can get a little grey of the ruler of a nation is someone like the Pope of the Church of Bane, where they are going to be a tyrannical bastard, but they might not necessarily be evil and might actually have the welfare of their people in mind. Another example is the Kingpriest from Dragonlance who was a good man with good intentions who still tried to commit genocide in the name of the gods. He's a good example of how PCs might oppose a priest of a good god doing his god's will. When you know for a fact that there is a god and he's on your side it might be much more tempting to follow the "kill them all and let Pelor sort 'em out" philosophy.
The catch with a divinely ruled domain is that the priest-king might be torn between protecting the souls of his followers and their bodies. The two goals are not always compatible. It would lead to an interesting balance where the king has to compromise as far as they can without losing divine favour. Another twist are pantheons: what if it's not a single faith that rules? This could be a united alliance of gods (the Sovereign Host in Eberron for example) or simply the dominant church of the time. The ruler might need to be a priest but they might be of any good. Imagine intra-church elections to choose a leader with all the other high priests acting as a type of cabinet or congress. A nation ruled by several (or all) religions.