When I first crack open a new book, one of the first things I want to know is: what kind of world am I entering? Will I like it? Will it be mysterious? Explore-worthy? Exciting? Exotic? Familiar? It's easy to tell within the first chapter how important the world-building is to the author of the story too. Not that books with fantastic worlds are the only ones worth reading, but they sure do make an impact. Think of some of your favorite movies for a moment. Characters aside, what worlds come first to mind? Why?
A good writer will not only construct an engaging world for their story, but that world won't be mere scenery. The writer knows how to use the setting in active ways and also makes the details of that world relevant to the story and the characters. The details they take time to focus on will reveal to the reader the thought processes of the characters, vital clues and foreshadowing, antagonistic forces, means of rescue, and the natural or man-made rules of that world.
A poorly thought-out world can turn readers away and may even create plot holes, writer's block, or other headaches for the writer. Sometimes what a writer is missing, that last crucial piece they can't quite figure out yet to make the story perfect, is complete world-building. The great thing about constructing a complete world is that it gives a writer more freedom to be original, or to create twists others haven't thought of yet. Say your plotline is pretty basic and even your characters and their situations have been done before. Your world-building might make the difference from being "oh, another (insert big name story or genre)" and get your story to stand out from the pack instead.
There are all kinds of worlds to create and the best part is you can make up your own rules. Enormous freedom comes into play when designing your world. Be cautious though when you sit down to write your story within that world. The writer needs buckets and buckets of information on their world. The reader needs much less. Give the reader only what moves the story forward, what enhances, engages, or acts upon the characters and story. Avoid huge block paragraphs of description, even if all the details you came up with are soooo cool. The reader doesn't need all of that. YOU do, however. Create a file and devote it to your world-building and research. Pour whatever you develop and need into it. Then learn to let go. You become the veteran traveler in your world and your job is to guide the characters and the reader through it. There isn't time or reason to show them every blade of grass or to explain all the principles as to why waterfalls flow sideways. Good tour guides know better than to overwhelm or bore the people on their tour. They know what to show and when in order to give the best tour possible, giving travelers something to go home and talk about long afterwards.
Be bold, not timid. Drive from your mind the conventions and comparisions to other peoples' work. Create a world you'd want to spend time in, something that fascinates, scares, enthralls, or seduces you. Think of ways your world can add complications to the plot or internal journeys of your characters. What details from this world will show your characters' personalities or quirks along the way? What is in danger in your world? What is right with it? What is wrong? And I mean wrong in a good way. The kind of wrong that the characters need to address and fix in order to reach their goals.
World-building goes so much farther than designing a good bit of back scenery for the characters to act in. We're not putting on a play with cardboard, paint, and plywood. Just as characters shouldn't be flat, neither should the world they live in. It needs to feel tangible, real, and believable. It also needs to feel important.
To give you a couple of jolts of inspiration, should you need it, first off I recommend E.F. Jace's world-building series for fantasy writers. And secondly, I have to share this link to the World's Coolest Staircases. I have an odd obsession with staircases, and some of these examples blew past even what I expected. A good slideshow to get you thinking "out of the box" or in this case genre or the most current popular worlds in literature.
Where do you turn to look for new inspiration when world-building? How much time and effort do you like to put into it? What is your favorite part of world-building? What world, designed by other writers or film-makers, have set your mouth agape or drew you completely in?