Verisimilitude
It's so important that it was written in the writing room of Superman (1978).
Making your world feel real is half the battle for a DM. It is strangely the most effort and the least commented on part of the work that you do. If you don't have an internal logic to the world then you run the risk of alienating your players. The thing I really hated about Harry Potter *SPOILERS* was when he saved himself from the Dementors. He went back in time to stop himself dying... from a point in time after he had died. It hurt my brain and took me out of the story. Later on it made sense because it's actually Snape that did it not Harry causing an impossible temporal loop...
Anywho, these little things matter. If your players are paying attention to the flaws they won't be paying attention to your carefully built story. When you are making anything for your world be it a dungeon, city or an NPC ask the five Ws: Who, What, Where, Why and When.
Lets apply this to an NPC:
Who are they? - A man named Denby Farrington
What are they? - An angry Guard Sergeant in the City Watch
Where are they? - In his office organising the guards shift rotations
Why is he there? - He was an officer in the army but he was shot in the backside by an arrow during a cavalry charge. He is bitter that his glorious moment was stolen from him He is in constant pain and it gets worse when it's cold.
When is he there? - Office hour are 9am until 5pm.
After that you fill in the gaps.
It is even more important for places as just dropping a city onto the map requires some thought.
Who lives here? - Racial mix of the settlement and their relative wealth
What happens here? - Local festivals, religious leanings and a specialisation to the city
What is this place? - What climate is it in, what is the surrounding area, what's the ecology.
Why did this place occur? - Is it a trade route/farming community/port/hunter's retreat/ etc
When was it founded? - How old is the place and who founded it
Players enjoy the challenge of being able to get specific things in specific places, this means you don't hand everything to them on a platter. Items will then have different rarities in different places so will fetch different prices depending on where they are. Tortles don't wear armour so their merchants are unlikely to stock armour, Dwarves tend towards heavier weapons so their forges will have more hammers, flails and crossbows than bows and polearms.
A rich merchant city may have all the basic items in the Dungeon Master's Guide at higher prices because the wealth of the area has attracted skilled artisans and that the area is quite a showy or fashionable place. The items all act in the same way but are jewel encrusted, ornate weapons that are suitable for a noble's hip at a Lord's banquet.
So now you are getting an idea for the minutiae that you should at least have an idea of when you're making places and NPCs but also how they would react to someone who bucks the trend. The element of a class system leads to social strata, intrigue and power struggles. This allows you to know how an NPC would talk to your PCs, how helpful they would be, what affiliations they have and what the party can do to change that. Also what the NPC has on them to steal (if you have THAT type of Rogue).
Make sure that their actions have either foreseen or unforeseen consequences. Killing a Green Dragon that is terrorising a town is great but if in the process of it their actions led to the local water supply being poisoned the local townsfolk aren't going to necessarily welcome them back as heroes. This isn't to say that your players should be punished constantly but they should be aware that they are part of the world.
The last thing is consistency. If you have house ruled something make sure that you apply that rule again if a similar situation arises. There are rules for a reason, if there is an action then there is a reaction, if the reaction changes the world is broken.
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