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Okay, maybe that's just me. But nuances of culture aside, worldbuilding in all its forms is something to consider seriously. It doesn't simply apply to high fantasy and sci-fi. Every story has a "world" where it takes place, conventionally called the setting, and it is one of the main components of story. Surroundings can work with or against your characters -- or both at the same time! -- establish mood, or provide pivotal information at critical points. Snowy mountain forests swarming with wolves and dark, broken down futuristic police headquarters present their own physical settings, challenges, and advantages to be explored. A primitive mining colonials and aristocratic British nobility will provide completely different social conventions and expectations for characters to navigate. Swords and archery fill a different need than laser guns or dragon's fire. Every component of your story that isn't plot, character, or theme is world.
If your writing a western, take time to examine the culture. If you have a city girl moving to the plains, know what was in style, clothes-wise and behavior-wise, for her and how that would clash with the styles (or lack thereof) in whatever social circle she lands in. If you're writing a historical romance, or even have a romantic subplot, know what was appropriate and what may be accepted in modern times but would be considered scandalous by both parties back then. Know what was too expensive for certain people to own and what parts of town people wouldn't walk through alone at night. Know what slang is too modern to be included. Know how teenagers acted, because I'll guarantee you that many modern historical stories get it so wrong it makes me flinch.
Do the same with modern stories, even if it's in a country you're familiar with. Unless you live there, don't assume. In the South you don't say Miss, you say Ma'am, and you absolutely don't say boy, because Miss and boy is what you would call your slaves way back in the day. It's different in the North. It's different in Japan, where surnames come first and there are honorifics galore. In Spanish you have different levels of familiarity for your pronouns, and which ones you use when depends on whether you're in Spain or certain parts of Latin America. Some US states have plates on the fronts of their cars, some don't. Different cities have different forms of government. Some small towns, large towns, or sections of cities are heavily populated by certain ethnic groups, often because of the proximity to disturbed nations, shipping routes, gold rushes, historical trade deals, and historical immigration trends.
All this boils down to, if you're writing about the real world, do your research. Talk to people. I've read some author's accounts of traveling to places they are writing about, and noting things like the smells, common weather, color differences, and slang. The movie Bolt has a wonderful bonus features piece about how the creators went on a road trip during pre-production, traveling to the different places the story took place and taking pictures and video, documenting the different qualities of light so the animators had enough material to work with. If you watch the movie closely, you can see how the light changes with their surroundings and how the creators worked with it to strengthen the mood and the story, instead of scattering scenes willy-nilly across America.
Beyond the basics of culture, also try to get a feel for basics of living in any setting. Where you get water, what you make clothes out of, what you eat and how it gets to you, how you keep warm, how you keep dry, where garbage goes, how much and what kinds of garbage you make... Getting these details wrong in a real-world story will be a raging sign of ignorance and laziness to people who are familiar with those details. When you don't consider them in a fantasy story, it can present the same problems, as well as making you miss out on some majorly cool opportunities. This list of questions covers most of your basic considerations.
While these are generally fairly important when you are building a world from scratch, my favorite part of sci-fi and fantasy is the culture. It's one of the reasons I simultaneously enjoy and am seriously annoyed by many sci-fi and fantasy stories. While it's easy to say the fanbase is just obsessing and overthinking things, wouldn't Star Wars be more interesting if we got to see deeper into the various cultures? The sociopolitical impact of major wars and the use of a clone army? How the collapse of the Republic affected the Outer Rim and Hutt space -- or how it didn't affect things at all, demonstrating just how vain it was for the Jedi and the Empire to think they were shaping the universe? Star Trek spent its time poking a cultural differences and exploration, how it could be beneficial or destructive. Stargate follows in the same vein, complete with the difficulties of politics on any world and the troubles of misunderstanding. There are tropes such as "Every Alien Speaks English" and "Puny Humans," when it would make so much more sense, and be so much more fun, if different species and sub-species and sub-cultures spoke different languages and humans were as terrifying to some species as the Skrulls or Darkseid or the Galra or Borg or Romulans are to us.
Basically, embrace culture and history and what it can do for your story in any genre. This infograph covers many of the obvious and not-so-obvious facets of culture. If you're a geek like me, go all out and start working out how the humor of one culture might clash with the preconceptions of family or justice of another. If culture isn't your cup of tea, make sure you at least have the basics of what will affect the story hammered out. If your MC is a thief, know where thieves stand in the social structure (it's not always at the bottom), how other people perceive them (the most horrendous of criminals? Dashing rogues?), and how they'll change their behavior based on the job (you don't act the same stealing from a fruit stand as you do from a mansion). Don't default your story's culture to the one that you're used to. Study other cultures, historical and modern, mix-and-match, come up with new concepts, theorize about how technology affects social status in the future and all the other fascinating things you can come up with!
To summarize: be creative, be realistic, and be thorough. Flesh out the world your characters live in and your readers will thank you.
