1) Simulationist. A lot of worlds are designed following certain rules or heuristics, and the point is to see what sort of world arises when certain conditions are assumed. Sometimes this is trying to simulate the growth of civilizations from the dawn of time. Other times it's more "if we assume they have access to X kind of magic or technology, what would happen?" Other times it's simply drilling down into an otherwise relatively common setting to try and explore what sort of conflicts and faults exist in our ordinary assumptions (deconstruction).
2) Personal. A lot of worlds are created just because you feel like creating a world yourself or think something is cool. You just want to see silver towers, lush forests, titanic space fleets, gritty cities in the dark, cyborgs leaping up to take out a helicopter... you want to create a world. You generally want to see specific events or places or things, and you create a world around that. These worlds are more of an exploration of your personal interests, and can often carry a vivid vitality. But, on the other hand, they can also be pretty standard - left to its own impulses, personal worlds tend to be very similar to the worlds that inspired us in the first place.
3) Evocative. A lot of worlds are created specifically to allow people to explore and experience specific values or moods. The aesthetic, rules, and situations that govern the world are crafted to create a specific feel. While a personal world might have a dark and sinister city, an evocative world would weigh the players down with that darkness and grit.
To give examples:
Batman's Gotham is evocative, at least at its core. Gotham exists as a heavy weight across all decent Batman comics, and the villains all arise from Gotham's aesthetic and tone.
Superman's universe is personal. It contains whatever villains and situations seem like they would be fun or interesting.
Watchmen is simulationist, exploring the fault lines in the evocative and personal comic book superhero worlds.
If we switch over to RPGs:
White Wolf's vampire games are evocative, built to
Dungeons and Dragons is personal, full of dragons and elves and whatever else pops up today.
There's not really any big-name simulationist tabletop RPGs, because those worlds tend to be more interesting to the GM than to the players. Maybe Traveller might qualify.
So, why am I pointlessly outlining some world design philosophies as I see them?
Because I want to explain a little bit about using pieces of them at a time. I think people's designs are stronger when they understand the strengths each philosophy has to offer. After all, any of those philosophies held too tightly puts out boring nonsense.
Let's take a Standard Fantasy World.
One big advantage of a standard fantasy world is that it is easy to understand. Players don't feel like they are confused: they always have a grip on the world, and therefore their adventures can be about adventuring rather than figuring out what the hell is up with this world.
One big disadvantage of a standard fantasy world is that it is contagious. Every world is poisoned by it - if you ever create any new species, they will immediately be renamed "adjective fantasy race/job". For example, "space elves" or "death knight". The only way around that is to make your world so far removed from the standard pattern of land-based politics that those ideas no longer apply. And that's a lot of work.
In a standard fantasy world or variant, the species are often boiled down to their statistical bonuses. Elves are +2 dex with forest-based flavor text. Dwarves are +2 endurance with cave-based flavor text. That's extraordinarily lazy.
Ideally, each race plays a particular role in your world.
Now, I don't necessarily mean that they have specific nations or policies or whatever. I'm talking about using them to express the foundational elements of your world.
If you are building a simulationist world, each species you plant should represent a specific aspect of the heuristics you're exploring. For example, if you're exploring the effect magic would actually have on a society, they would represent different responses to having magic. Maybe the elves teach magic to everyone but have very strong restrictions on learning magic past that level, maybe dwarves outright ban it, maybe humans keep it to the upper class and don't allow the lower class to use it. On the other hand, if you're exploring different categories of magic instead of the effects of magic on society, each species would be one category of magic. Whether that means air-earth-fire, or whether it means something more unique, like summoning-enchanting-weather control.
If you are building a personal world, each species should represent a particular favorite piece of the fantasies you are trying to include. The dwarves are the endless rows of dour warriors taking up arms. The humans are the flashy solo fighters that revel in high adventure. The elves are the calm, deep waters of ancient magic.
If you are building an evocative world, each species should represent a piece of the mood you aim to create. If you were aiming to create a White Wolflike depressing world of fantasy, the elves could be the oppressive high lords. The dwarves would be the endless underclass. If you were aiming to create a dreamlike primitive world, the elves would be the ancients who barely touch the mortal world, the dwarves might be just figuring out farming, and the humans might be migratory hunter-gatherers that prize their spiritual connection to the wild.
Using races to express foundational elements serves three big uses. First, it gives players a clear grip on the foundation of the world. They'll understand the sort of situations that are going to arise and the sort of tensions that exist. Second, it allows players to affiliate themselves with a particular foundation and explore that element more personally by choosing to make a character of that race. Third, when you understand any given situation, you understand how it can go bad - this will allow you to create more powerful story arcs.
Now, you can also use non-racial expression, but there's always some kind of faction-choosing going on. For example, you might make different schools of magic represent your foundational elements, or different philosophies, or different alignments. The key is that you can give your players a good grip on the world you've created by making it easy for them to see where you've drawn lines.
Once you've sketched out your world in your head, you'll often want to fill in details. But this is a bit fraught. First, you'll often be filling in details nobody will ever care about - it might be better to give the world a spin and fill in the details as the adventures unfold. Second, you'll often fill in the details willy-nilly, rather than thinking about the way it helps the world or the story or the player's grip on those things.
This is where understanding the strengths of the different philosophies can really help. Each philosophy can step in when you're filling in details, and give you a rich and varied world that never feels erratic or confused.
Simulationist philosophy brings the idea of connectivity. Events are connected to events, people to people, places to places. And, of course, people to places, events to people, places to events... you can always think "what sort of thing would arise in this situation?" and "what would happen because of this?"
If you use it too much, it can become a litany of this-then-this-then-this writing, which is the most boring writing style imaginable. But, when used sparingly, it can show that small events have big results, and big events affect small people. It can also be used to establish clear connections in the world, giving players a clear direction to explore and plot arcs a clear direction to spread in. And, of course, it can highlight cracks in the world that are due to be exploited by villains...
Evocative philosophy brings with it the idea of staying on-message. You want to make the players feel a specific way? Then the world needs to be built to make them feel that way. You can't simply write "this is a big evil city". What sort of day-to-day situations make it evil? What sort of architecture? Social structure? People? Give the GM plenty of tools to weigh the players down with the mood. Unlike simulationist stuff, this is more a list of options rather than a chain of commandments, so even if you provide too many options, the GM can just use the ones they like.
Another big tool evocative philosophy provides is the mode switch. If you're writing about a city, write about a person in the city. Write about a street in the city. Write about a company in the city. Write about how the city interacts with other cities. You can easily keep the same tone and create something in an entirely different scope. This is vital if you want to have the players actually feel the mood: sticking to one scope will be too limited and the GM will have to fill in the other scopes with whatever happens to pop into his head.
Personal philosophy brings with it the idea of beats - specific cool things that happen. What would be cool in this situation? Write it down. Don't try to justify it or say it already happened. Just give it to the GM as a suggestion of something cool.
If you're creating a dystopian cyborg world and are outlining a downtown area, just mention in passing "Jumping from a rooftop to the top of a VTOL enforcement craft has a difficulty of 12". If you're creating a fantasy world of high adventure and outlining a forest, just mention "the forest is full of little gullies and hillocks, which goblins tend to pour over suddenly if the adventurers stumble into their trap."
Anyway, those are my thoughts when I design worlds.