Where you start running into problems is when you want to let players define the game they want to play or where you want to run a game outside the box. So lets say you have:
1) "Let's play a game where we are all pirates!"
2) (Inspired maybe by "Spice and Wolf" or "Lord Valentine's Castle" or "Station 11" or "Firefly") "Let's play a game where we are travelling merchants."
3) "Let's play a gritty survival game, like we are all stone age tribesmen!"
4) "Let's play a game set on floating asteroids in a vast nebula where we fly around on flying mounts!"
5) "Let's play a dynastic game where we are all feudal landholders who are vassals to the same lord."
Or any number of other things where you have a major component to gameplay that isn't dungeon crawling where mundane everyday life isn't so mundane and is in fact pretty dangerous and potentially fulfilling as an adventure fantasy.
And you might say, "Well, you are playing the wrong game.", but that implies that I know ahead of time exactly what game is going to emerge or perhaps more importantly, that the same long running campaign can't feature all of the above based on player choices at different points in the campaign. Because you know, I've been in a game that diverged across that many different types of gameplay in a single campaign/story without necessarily all of them being planned by any one participant (including the GM).
The problem isn't that I want to push "Papers and Paychecks" on my players, but that in the course of play I never necessarily know what the players are going to seize on as the fun of that moment and even in my own plans for the game there can be periods where I might really seriously care about the mundane everyday hazards of disease, heat exhaustion, and wear on the body from marching through terrain humans are ill adapted to in addition to "eaten by dinosaurs".
Quoting myself here because what I thought was obvious is apparently not obvious.
Let's say you want to play a game of pirates. If you pursue that game for any length of time so that the players are successful pirates and achieving the sort of aesthetic that people have in mind when they think about pirates, and in particular if the players know anything about pirates and that's why they are excited to play pirates, then you will eventually find yourself in need of a few things:
a) Detailed rules for nautical vehicles.
b) Some sort of mass combat rules for handling abstract combat with scores or hundreds of characters on each side.
c) Some notion of how character skill relates to all these things - sailing a boat, repairing a boat, swimming, navigating a boat, etc.
d) Probably eventually some sort of rules for handling underwater combat.
If your system doesn't provide expansion into those areas of play with supplemental rules that most people may never need, well, then you're going to have to either do a lot of fiat or make a lot of rulings and rules smithing on your own. But worse, if the system has shoddy rules for these things because the designers didn't think they were important and so spent no time on them, your experience will possibly be even worse because you'll be misled into trying to use the RAW and won't realize the problem until you abandon this approach in frustration.
This is the story of my gaming experience with D&D from about age 12 to age 17. I kept trying to apply the rules and examples from the rules text to the scenarios that I wanted to run, only to find that things weren't working. From about 17 to about 23, I spent a lot of time abandoning the RAW and trying to house rule the game, often based on increasing exposure to other game systems. Both of the major groups I was involved with in these periods took it for granted that all the problems and frustrations we were experiencing could be summarized as "a lack of realism", and if you look at the above needs and the above attempts to fix them this assumption - though wrong - becomes more understandable. With the rules not working for us, a lot of time was wasted in arguing what would be the realistic result of a proposition. A lot of time was spent trying to deal with the silence of the rules on character skill in substituting player skill and knowledge for character skill in knowledge, would lead to further arguments about whether the player's detailed descriptions of what he was doing were actually realistic solutions to the problems or whether this was metagaming because it wasn't clear how much of this the character should know.
Further, if you really want to play a game of travelling merchants, you have to now start dealing with modelling an economic system. While theoretically you might have encountered this playing pirates, and pirates also have booty to sale and property to maintain, you might be able to ignore this with having booty consist of generous chests of gold doubloons and jewels as with more fantasy pirates. But once you are travelling merchants now you need:
a) Some notion of the cost of production versus the market price so that you have some idea of profit.
b) Some system for negotiation of prices.
c) Prices for a whole lot of things that are not adventuring gear and more importantly very strong and well thought out prices for everything that will work in a simulation, and which aren't set by various gamist balance issues as they often are in D&D.
d) Possibly some idea of abstract maintenance costs so you can calculate overhead.
And again, if your chosen system ignores all this you will be out doing research and making rulings and hopefully you are a decent rules smith, but worse if your system has shoddy versions of all this you're going to be badly misled and produce all sorts of unreasonable results and you probably won't figure that out until you've gotten really frustrated.
Now, if you decide to play stone age tribesman you end up in a different situation. Here the most overwhelming difference between the game you play and the default setting is that there are no stores. It's impossible to buy most anything and trading is unreliable.
a) So immediately you find you have the need for a crafting system that produces reasonable results.
b) You probably also need to have some rules around much finer granularity of quality of goods than just 'normal' and 'masterwork' because you can no longer assume that any tool in the environment is made by a skilled craftsman working from centuries of learned design using quality materials.
c) You probably also need travel exposure to the elements rules that actually work and don't require a dozen rolls per day to determine just a single thing and "man vs. nature" is probably going to be a big part of your gritty aesthetic.
Same problems apply.
And if you are playing a game where flying is a thing that happens all the time, then aerial combat and movement starts to become a thing that you need to think about. You end up with questions like:
a) If we have gravity, what's the maximum safe climb or dive rate?
b) If we have gravity, what's the minimum forward movement rate to maintain lift?
c) How much can you adjust your direction of travel in a round? If your rules set normally assumes facing is something that doesn't need to be tracked because everyone is either on the ground 99% of the time or has perfect magical flight, this last question can now prompt you to imagine how facing works within your system which brings up things like what weapons you can employ depending on the position of an opponent.
d) What's the aerial equivalent of acrobatics or athleticism and what does that do for you in terms of opening up flight maneuvers you might not otherwise have?
e) One of the things I find you often discover once mounts and vehicles are ubiquitous is that you suddenly do find yourself in scenarios where you need chase rules because your tactical turn-based skirmish rules meant to work well on a smallish map where no one was trying that hard to get away from you don't work so well when everyone is continuously moving in the same direction.
And if you get into a dynastic game of feudal landholders, I wouldn't be too surprised if almost all of this comes up in some form or another and in addition you start finding that you need rules abstraction to represent economic and political holdings, heredity, childbirth, and education of children, servants and vassals plus mustering armies and levying taxes. And somehow you need to keep this simple enough that it's playable while having enough verisimilitude that the answers just don't seem stupid or game breaking.
Different game systems do different parts of this well. For example, Traveller, Pendragon, and Blades in the Dark all have some interesting rules because they considered from the start parts of this to be their core gameplay. But my experience in 40 years of doing this is that if you run a campaign for 4 years or 10 years, you are going to find that regardless of what your core gameplay loop is, you're going to find yourself branching out into all sorts of other areas. And when you do that, you can really appreciate when the game designer has put some real thought into their system.
1e AD&D was struggling with all of these things both in its core rules and to a larger extent in the pages of Dragon Magazine. And I can imagine the difference between how 17 year old me would have been able to react to some cool idea that struck me at the time using the tools I had available and the tools I have now. Like for example, suppose my idea for a campaign hook was a shipwreck in a storm in which the PC's were trying to rescue the survivors, and a threw out a pitch to my players, "Heh, guys I got this great new campaign idea, and it all starts with the background that you're all residents of this small coastal fishing village." Using the 1e AD&D rules as a toolkit such a pitch would in no way suggest anything to my players or give them any way to meaningfully respond. But if I make the same pitch to players aware of my 3.0e D&D inspired rules all sorts of things just float to the top as possibilities to inspire the imagination and the rules themselves suggest all sorts of ways of mastering that scenario. Those characters would actually be competent and would actually have backgrounds and could actually cope without fiat assumption of coping unlike the 1e AD&D characters even though both of the characters were 1st level.
The 1st level character would have "a lot of past in their past" without needing the assumption that that past was heroic adventures.
The difference is that the 1e AD&D rules were largely silent. And it turns out that silence in the rules doesn't actually inspire the imagination. The general response to silence is to ignore it and to focus on what the rules actually say and to become deaf and blind to what your game is therefore missing as possibilities.
