I've been thinking about world design, especially scifi world design.
Most scifi worlds seem to include flying cars. It's becoming steadily less popular, being slowly replaced with the idea of merely "hovering" cars that are just like normal cars except without wheels.
I was thinking about why flying cars never came to fruition. You can argue technology, but I think that even if we have enough technology to create them easily, we still won't see them, except on worlds where gravity is below 0.3: it would be difficult to get enough ground traction to move your mass at that point.
Anyway, I think the reason we don't see flying cars (or flying anything) is because of how we build our cities. As humans, we tend to centralize. Because of this, it's pretty easy to build roads from centralized point A (your suburb) to centralized point B (downtown offices). And it's clear that it's actually easier - more efficient, more effective, and more psychologically acceptable - to improve the roads rather than the vehicles we drive.
I can see a world where flying cars are a necessity, but it would have to be a world where the places of interest aren't connected by roads. Basically, a world of hermits.
In my mind, world building is perhaps primarily about movement. How people move from place to place, how information moves from place to place. By changing how often and how rapidly people/information moves, you can radically change the feel of a world.
In a world with flying cars, most people would probably not travel very much. If they did, they would travel to be nearer each other. Flying over to visit your friend would be a weekly thing, and you'd probably produce all your food in-house. Now, there are two fun things you can do to this world.
One would be to make information flow very easily. Although everyone is separated in distance, wireless communication is extremely easy, and everyone is pretty much always talking to each other over some extremely high bandwidth VR system. In essence, although they don't physically travel, they cohabitate as they want. They may even use robot proxies. It's a very social, very connected world, minus the actually being near each other bit.
That kind of world would have a very different feel from one where some technobabble keeps wireless to a minimum. In such a situation, communication would be very difficult, perhaps even restricted to people physically driving their flying cars around. There would have to be some reason why everyone doesn't just move next to each other, but assuming there is, this kind of world would lend itself to a more exploratory game, trying to find another hermit's dwelling, trying to figure out what's going on with only a trickle of information...
This kind of thinking also works in fantasy settings. For example, signal towers are a great way to rapidly send messages from place to place: a nation with signal towers will be much more organized and collected than one that uses riders. You can put in fun details: the signals can only clearly be seen at night, so they don't operate during the day. In turn, this means that any time anyone wants to do something particularly dastardly, they do it at dawn, so they have a maximum amount of time to run before the signal towers report their crime...
Dragons are another example. Usually, dragons are used as combat mounts, as if they were helicopters. But that doesn't make much sense: economically, you can make a fortune hauling highly perishable goods to the wealthy. Mail, spices, ice, exotic fruit, vacationers... assuming your world has enough wealth to feed the dragons off profits, you could do very well at that sort of thing. But no matter how fast they are, dragons are slower than light, so it doesn't make much sense that they would be the communication medium if your culture has thought of signal towers.
To me, these kinds of details are what make the world tangible. If you have a fantasy setting, we're already bored of it and we haven't even seen it yet. On the other hand, if your next scene is a tremendous dragon landing in central plaza, watched by a horde of eager peasants, we're suddenly interested. A cry of "mail's here!" would make your world particularly fresh and unusual.
The reason I focus on travel and communication as my favorite aspects are because I've gotten in the habit of designing game systems around the idea that the players will primarily want to travel and communicate (usually with each other). It's naturally wandered into my thinking about world design.
What kind of thing do you think of to keep your settings feeling fresh?