My old Namea campaign was designed around the idea that societies had evolved with D&D magic and other intelligent races. It was very educational in as far as campaign design - here are a few things we found were good to keep in mind.
1. Abusive magic: There are a whole lot of spells and spell combinations that can literally dictate how your societies will be built and defended. Teleport, Gate, Shapechange, Scry, Earthquake, Instakill spells, Control Weather, Disjunction, Wish, Fly, Invisibility, Domination, Time Stop. There are specific counters to many of these to protect small locales, but not entire villages, towns or cities.
2. HL/EL characters: The most important question will be: What are your HL demographics? The difference in power and capabilty between commoners and HL characters is staggering, nothing in real life compares. HL characters can possess Intellects, Wisdom and Charisma that we can't even imagine IRL. Their capabilty is awesome, they can do so many things so well that a small band of retired adventurers could do the work of hundreds, if not thousands of commoners - war, agriculture, manufacturing, etc. Skills such as Diplomacy, Perform and Intimidate can bend people to your will (even enemies) through a single check. These are things you might want to address (nerf or otherwise).
You also have to accept that HL characters/monsters can simply kill an unlimited amount of commoners and living in a D&D based world and life for the average commoner might be very scary indeed. All these factors are going to influence how commoners behave and their outlook on life.
3. Monsters: If you're dropping down 2 or 3 monster manuals and saying "All these races exist on my world." you should also consider how the PC races managed to survive when you've just dropped 2 or 3 books of far more worthy races on the table. You can always institute the "divine right" clause but then understand what the gods must have done (and must continue to do) to suppress all the other races from achieving dominance. Spawn creating undead are like a virus that has to continually be kept in check, certain EL monsters can wreak absolute havoc on a campaign if allowed to exist, etc.
In our Namea campaign, humans ended up playing (in public) the neutral third-party race that was a go-between for the dominant species of the world (the beholder empires, the undead empires, the illithids, the giants [who supported the humans], demon-infested lands, etc.). They secretly worked to undermine their societies and towards keeping safe the last of the endangered PC races (the elves, dwarves, etc., and the last of the dragons).
Societies: How has magic altered the way societies developed? What kind of cities do they live in? Has it changed agriculture? Has it changed warfare? Economy? Political Structure? Transportation? Communication?
Give us an idea of "how much" magic you'd like to incorporate into your campaign and it'll be easier to answer some of these questions.
Cheers,
A'koss.
