I start with a 'hook' - this is usually a genre element that I want to play up for the campaign.
Let's say I pick 'airships' as my hook. Then I create a world where airships will play a major role in society.
On the one hand, this could mean that airships are new/prototypes and that the first one to launch will play a central role in the storyline. What role? Perhaps the creators of the airship are a small nation with late Rennaissance tech, threatened by a considerably larger nation or coalition with lower tech but greater numbers. The airship, with its immense strategic significance (it's of the floating battleship variety, complete with bombs), represents the turning point in the war.
This leads me to develop the factions involved. On the one hand we have a small country, higher than average tech, perhaps with a more scientific worldview. The default assumption would be to make them the 'good guys' because they're losing - so instead, I make them the bad guys. Like the Nazis in WW2, they thought their superior tactics and equipment would allow them to defeat all their neighbors, but they ended up biting off more than they could chew and the counterattack has broken their ground forces. The airship is their 'ultimate weapon,' their last-ditch attempt to reverse the war's course and resume their drive to world domination.
I'm getting a picture of a driven people, highly organized, progress-minded, convinced that their philosophy is worth 'bringing to the world,' even if they have to bring it by force and conquer everyone to do it. Their leader is a mad scientist type, immensely intelligent and charismatic as well as completely nuts. With his ideology of enforced enlightenment, he's an antagonist the players may feel some sympathy for - even as they want to smash his face in with a steam-powered hammer for what he's done to their characters in the name of his ideals.
The rest of the world, in contrast, is lower-tech, more late medieval or early rennaissance, insofar as the two can be separated in any meaningful way. More magical/spiritual than technological, perhaps with a strong unifying faith that the antagonist is opposed to. Most of the allied countries probably boast a feudal system complete with knights who have good reason to wear full plate. So far, they've successfully counterattacked based on numbers and personal courage/skill - until the insurmountable power of their enemy's technology puts them in grave danger.
For the airship to be so dominant, domesticated flying creatures (griffons and giant eagles and the like) will have to be excised. Likewise D&D-style dragons with their inborn 'firepower.' On the flip side, wingless land dragons or sea dragons could still have a role. In general, monsters won't play much of a role in this campaign, with most being one-off 'magitech mutations' unleashed by the antagonists or summoned celestials/spirits bound to the protagonists' church.
From this basis, I would detail the regions the PCs were actually operating in, perhaps even getting away from the airship hook as anything other than a looming menace/final dungeon.
Now, even though this whole world is built up around a single genre convention, I could easily develop it in such a way it could be used for multiple campaigns. It would be easy to tweak a few elements to make it playable from either side, particularly by setting it at different time periods. A post-war 'inquisitors hunting down renegade engineers' game with the PCs being survivors of the defeated technocracy (or inquisitors hunting same), for example.
For that matter, the genre convention 'airships' could easily go in entirely different directions, say, by focusing on a whole world where airships were the main means of travel. Floating continents, perhaps? Or islands stranded by a greatly increased sea level?
Anyway, that's my world-building process of choice.