Your mountaintop archipelago world reminds me of other sci-fi storylines as well- Niven's Integral Trees stories and Hal Clement's Mesklin stories, collected in the book, Heavy World. Waterworld also springs to mind.
Like the Integral Trees stories, one of the limiting factors to any culture will be scarce resources.
1) With the high-points split up into a myriad of peaks like a chain of islands in the Pacific, your people are going to have a devilish time getting fresh water. This is assuming (as I believe) that high-altitude air can't hold as much H2O, which will translate into less precipitation, and thus, less easily retrieved water.
2) Nutrition in general will be more difficult. (Think of Easter Island or pre-freeze Greenland.) Some peaks might not be big enough or fertile enough to support a wide enough variety of flora and fauna to support long-term settlements. This could lead some settlements to have harsh population controls, enforced by government (Logan's Run) or religion (any faith involving ritual sacrifice).
3) If you have races that are miasma-native or miasma tolerant, you've got the basis for trade and conflict similar to fictionalized accounts of interactions between Atlanteans and "Surface Dwellers"...or Elves and Drow...or Eloi and Moorlocks.
Like on Mesklin, travel between settlements will be difficult, but with potentially high rewards for intrepid merchants.
1) Anyone who comes up with a new and reliable way to survive and navigate the miasma has an immense "technological" advantage.
2) The isolation of the settlements will mean that you'll have varied rates of tech and magic advancement, linguistic divergence, and possibly even genetic diversions & bottlenecks (see Darwin & the Galapagos for a refresher).
Divergent tech/magic could include things like:
1) Hot air baloons or dirigibles for travel. Perhaps some settlements have access to plant material (like Liftwood from Space:1889), a mineral or alloy like Cavorite (like in First Men in the Moon) that makes heavier than air flight possible. These would be resources worth fighting for...
Other artificial flight methods could include aircraft deriving lift from enslaved or allied air or fire elementals, working ornithopters, or even sophisticated gliders.
As a potential "megastructure," perhaps someone is building or has built a skybridge similar to a rope bridge but with a system of baloons/dirigibles supporting some of the spans over deep and wide chasms. It couldn't support much more than foot traffic and pack animals, though- no cargo or war vehicles would work due to weight.
2) Using open portals to the Plane of Air to supply additional oxygen. This could mean added land for a settlement and easier mining of resources. Such a portal could even be used to pump air into RW "diving suits" or something more akin to those in 20,000 Leagues Under the Sea.
3) Some alloys and materials may not be universally available. In short, you have the basis for a DarkSun type economy.
4) Perhaps someone successfully makes an "airtight" vehicle that carries a smaller portal to the plane of air- essentially a "submarine."