Let's say the adventure is to go into a dungeon and recover a relic of some sort. The party starts in a nearby village and has to travel to the dungeon. The DM has done no worldbuilding beyond what is present in the adventure. He is also not gifted at improv. I as a player have some questions:
ME: Ok what nations are there? Where can my character be from?
DM: There are nations for all of the races in the PHB.
ME: Ok...what are the human nations like?
DM: I don't know...what kind of nation do you want to be from?
ME: Well, can I be from one that is kind of like ancient China?
DM: Sure.
ME: How does my nation get along with the other nations?
DM: That isn't important in the adventure.
ME: Ok.
So I make a human priest and do what I can to make him represent a culture based on ancient China. We start the session in the town. Again, I have some questions:
ME: Is there a branch of my church in this town?
DM: The adventure doesn't say...I'll say no. It's too far away.
ME: Ok, are there any churches in the town?
DM: Just one.
ME: Ok, I go there. I'm going to talk to the priest to get a feel for his religion.
DM: It just says his name, level, and that he is a priest of an agricultural deity.
ME: Ok, my religion venerates nature spirits so we should get along well.
DM: Sure.
ME: Is the town facing any problems that I could help with before we head to the dungeon?
DM: It doesn't say...so, no.
Darth Shoju wrote that about a decade ago. It was a strawman, but at least it provided a starting point, for others to counter with other ways that session could have gone. For better or worse, for richer or poorer, with the DM or the player to blame, because too much worldbuilding or not enough.
For this thread to successfully and usefully "WISE FWOM YOU GWAVE", maybe we would benefit from updated strawman examples. Or better yet, actual examples, from "My DM did this and I walked away", to "I did this, and one player walked away, but the rest of them loved the campaign."
In the meantime, D&D now has a 5E PHB, and the passage on humans lists nine human ethnicities from Forgotten Realms! So *already* we have divergence on the outcome which results when the DM says "Just what you see in the PHB, pal".
My $.02:
"ME: Ok...what are the human nations like?
DM: I don't know...what kind of nation do you want to be from?"
Good jorb, DM! You expressed interest in what your players want! Which is not the same thing as always answering YES. (No, you cannot haz a phased plasma rifle in the 40 watt range.) Was it really necessary to point out the difference between "showing interest" and "always saying yes, to anything"? You tell me.
ME: How does my nation get along with the other nations?
DM: That isn't important in the adventure.
Partial credit, strawman DM with neither worldbuilding nor improv skills. Alternative answers include...
DM 1: You are from so far away, that the topic will not arise, unless some other player wants their character to be from the equivalent of ancient Japan. If so, we'll take that as the baseline for how you get along.
DM 2: That's currently a blank page. Do you have an interest in establishing personal trust despite national enmity, or something like that?
DM 3: Everything changed, when your nation attacked.
(The ancient psuedo-Sino-nation is now the Fire Nation.)
ME: Is the town facing any problems that I could help with before we head to the dungeon?
DM: It doesn't say...so, no.
DM 1: They're worried about that relic. The sooner your party finds it, the better. If you don't, maybe it could fall into the wrong hands.
(DM 1, writing a note to self: BBEG is on the trail of the relic. He will arrive in two days. I'll stat him up later. If the PCs emerge from the dungeon on Day Two, then "Again, we see there is nothing you can possess which I cannot take away. And you thought I'd given up.")
DM 2: "That's also a blank page. Hey, other players, anyone wanna suggest a problem?
READY PLAYER TWO: "One of the villagers is sick."
DM 2: "Yeah, we'll go with that. Did you prepare Lesser Restoration, and if so, do you spend the spell slot?"
DM 3: "He says they have no problems. They have no problems of any kind. They have never had any problems of any kind. Everything is fine. Everything is JUST FINE. The local cleric then points out that it's time for you to go, so you can reach the dungeon before nightfall."
(DM 3 to self: Maybe I'll think of a problem later.)
READY PLAYER TWO: "Roll insight!"
DM 3: "You're not there. This is cleric-to-cleric professional courtesy. Wait your turn for the spotlight."
As you can see, DM 1 is "Get back on those rails, I got a dungeon for you to explore", while DM 2 is a heavyweight sand-boxer, and DM 3 is my version of the happy medium.
So... how could this be improved by worldbuilding?
Or is this the optimum D&D session, which has successfully averted all the problems which follow from worldbuilding?
Or something else?