One side effect of crawling through the huge pile of TSR era modules over the weekend was rediscovering how many awesome lost world modules are out there that I've never run. Dwellers of the Forbidden City and Isle of Dread are really excellent - but for my games, the tone usually isn't right. Why not? I started brooding on it. Here's what I've come up with - there are two general paradigms for a D&D campaign, one with a local focus where adventure is all around the home base, and one with a cleaner line dividing civilized lands from the wild lands - I'll use the terms Points of Light and The Wild Frontiers.
Points of Light
Everyone remembers the term Points of Light from the 4E launch, when grognards everywhere said, "Big deal, I've been playing a Points of Light campaign since the 1970's". The idea is that each town, village, or home base is a little island of civilization in a sea of darkness - monsters and danger are everywhere.
In the typical D&D game, the local farms are harassed by kobolds and goblins in the woods, there are bandits on the roads, and the kindly traveling priest is actually an agent for the Shrine of Evil Chaos, spying on the villagers. No need to go far for adventure, it's all around you!
Most D&D settings have post-apocalyptic or fallen empire themes to explain all that gold in the ground and those magic items littering the ruins. This works well for dark ages or early medieval settings, standard fantasy settings like Greyhawk and the Realms, future earth like Xothique, and ancient world/sword & sorcery games with isolated city states (like the Wilderlands).
The other approach is to have an extremely civilized and mundane homeland "back there", and put the campaign firmly on the wild frontier - far from civilization, across the ocean, or on another continent. The (famous on the internet) West Marches campaign followed this approach, and James Raggi's theory of Weird Fantasy in LotFP champions this approach. This paradigm assumes a degree of stability, trade, and commerce back in the homeland that has pushed the frontier further out.
Games set in a late medieval period, the Renaissance, or the Age of Sail would fit this theme well; so would an ancient setting featuring something like the Pax Romana - the borders between the realms of Law and the wild frontier are clearly defined.
The Lost World Scenarios
Now I begin to see why adventures like the Isle of Dread, Dwellers in the Forbidden City, or the Hidden Shrine of Tamoachan haven't been at the top of my queue - I usually run Points of Light style campaigns. There isn't the social infrastructure for merchants or rulers to outfit seafaring expeditions to cross the ocean and exploit the Isle of Dread or explore the Forbidden City. Adventurers in a Points of Light game are more concerned about the giants in the mountains, the sleeping dragon in the old ruins, or the rampaging orcs of the dim forest. Why go to the big city and outfit a sketchy expedition to go far away, when there's a vampire in a castle right there on the map waiting for someone to kick him in the fangs?
If you make the homeland "mundane and civilized", then the party has to look for monster-bashing adventures out on the frontier - whether it's an island across the ocean, a distant continent, the center of the earth, or a lost world, there is a journey involved. That whole paradigm just screams pulp action, doesn't it?
The theory has implications for The Black City setting, too - the discovery of the frozen island of Thule implies an environment where ocean travel and exploration of far away places is common. (Bless the Vikings, they break all the rules).