I just got into a spirited discussion about economies in world-building (either for a game or just as a setting). The discussion revolved around what factors are important - food, energy, building materials, etc, etc. I quickly derailed it into a pages-long argument for transportation being the only thing that really matters.
This post is pretty much a technical description of why I say that, and what my experiences have been. If you're not into world-building, skip this.
When I'm world building, regardless of technological level, the first thing I think of is transport. How do goods and information get from one place to another?
Some of the people arguing against my position take the position that, at least in high-tech environments, transportation is cheap enough to disregard. However, regardless of price, transportation is what enables the growth of economies. All economic centers are also transportation hubs. This is a symbiotic rather than a causal relationship: a stronger economy promotes more import and export, and more import and export promotes a stronger economy. This is not simply a function of infrastructure, either: it's a fundamental matter of how humans think and plan.
Having built many worlds with this philosophy, I can say that it allows you to quickly and fluidly generate a dense and real-seeming set of nations and cities, regardless of technological level or what economic rules you have underlying the simulation. This is because almost everything about a city and nation is heavily influenced by how many transport routes they have running to/through them, and how open/closed/taxed those routes are.
Things like war, disease, and the spread of culture all happen along transport routes, all of which are marked by a size denoting their max load. Size is not length: it's the amount that can be transported in a time unit, which includes not just the actual road/river/spacelane in question, but also the docks on the tips. Safety can be assumed to be a function of size. Things like strife and war can also be fairly reasonably simulated using nothing but transport lines to determine how nations feel about each other, as long as those transport lines include a rating not just of size, but also of taxation/closedness.
Example: it's ancient times, farming's just been invented, and we use a random seeding system to create tiny little villages. After that, everything is determined by the trade routes. Rivers and coasts are easy to transport along since you only have to build some docks, and roads can be built if you take an economic hit for the initial investment. Not a certain amount of money, but a decrease in your "economic rank".
Speaking from experience trying to simulate a China-like environment, what we find is pretty realistic. Cities rise up on rivers and deltas and coastlines. A network of roads springs up as well, but inland cities never get very far. Not because they lack resources, but because cities on rivers and oceans have a strong head-start by then and dominate the route-building game.
In late game, cities will pop up on land routes simply because the land route is there. This, too, is realistic. Add in a random assortment of disasters that can damage trade routes, and you get a pretty reasonable feeling of organic complexity.
Adding some political acumen to the game works flawlessly, and you can see the borders form and wars start... based almost entirely on the idea of jockeying for transport hubs and routes. A second layer of restricting transport along certain routes or to certain countries further extends this political spectrum and, at this point, it becomes close to realistic. Adding in a dash of mutating culture and you have something that can pass muster with ease.
Of course, this can all be done manually, as well. I find most people manually building worlds end up thinking in terms of transportation, anyway.
A high-tech example: you have FTL drive and are all about building colonies on planets. You rank various planets with a basic "economic value". It's not even important whether the value is food-based, ore-based, or scientific-oddity-based.
The cost transporting world-to-world within a star system is low and flat, the cost for transport between star systems is per light year. All transport must be along defined "routes", but defining a route is cheap, consisting of building a landing dock.
With these simple rules and a completely random star/planet layout, you will quickly find a dense web of trade routes that ends up almost identical to the one in our primitive version. The biggest difference is that we're now working in 3D rather than 2D. The difference is subtle at the moment, but if we add in the "upper layers" of the simulation to get political, the 3D world makes for some very odd political maneuvers.
If we add in wormholes for very cheap long-range transport, wormhole star systems become the most valuable, and almost instantly become massive centers of commerce: just like you'd expect, when you "cheat", you win.
Let's turn around and say that you don't need to define routes: any route is fine. This is actually unrealistic, since established trade routes are more than just a landing bay at each end. An established route is important for insuring company longevity, so companies would naturally start to form specific trade routes even in the absence of any physical need to. It's the nature of trade.
But ignoring that, even when treating every shipment with its own independent path-finding, we still end up with trade routes as they follow the same best path every time. The differences are actually pretty minor, although they grow more extreme as the simulation continues if you continue to disregard things like policing routes, taxing/banning trade, and so on.
Another factor that can be fun to include would be information transport routes. This is especially fun if the information transport system is fundamentally different from the physical transport system, ending up with a complicated "two-map" set of trade routes. Either way, it's a lot of fun if we presume that economies are two-stage: initially, goods are what matters, and the physical trade routes are stressed. As the economy grows past a certain point, goods plateau or even begin to decrease in importance and information creation/transport rapidly grows more and more important (exponentially). Info transport and creation can also be the primary source of technological and cultural innovation.
ANYWAY, this post is all about my pet peeve, so it was a bit dull. The basic lesson is: if you ignore transport in your world-building, you are ignoring a major source of depth.
What's your experience?