To finish up my musings about settings for a setting representative of the Basic/Expert game, I wanted to touch on just a few more topics: Resources, Risks, and Rewards.
For me, and many grumpy old-schoolers, managing your resources is no small part of classic D&D play. Heck, I even created products specifically to make it easier to incorporate into tabletop play. Rules like encumbrance, searching times, movement rates, and light source duration all lead to some real cost/benefit decisions being made: Do we take the time to search every room? How many torches did you bring? Do we hire someone to carry our extra stuff?
Now all these sound more game mechanics-related than setting, but bear with me. A BX world is a place where not only do decisions like those above matter, they matter because it's a world where dungeon crawls are a relatively common. Ancient labyrinthine ruins, extensive subterranean caves, mysterious catacombs, they practically flourish in a BX world. Of course this is true for many other versions of D&D, too. The point is the characters live in a world where someone, at some point, decided that it was a good idea to carve out an underground lair that looked like this.
There are weird, even zany places in a BX world. Maybe they are ancient ruins or a mad wizard's tower, but those that decide to brave those places prepare for mapping long corridors, regular booby traps, hidden passages, and foul monsters lurking around corners. Which brings us to...
Whether it's claw, sword, or spell, PCs face most of their risks in combat. The BX world is one where monsters are real and your character is going to have to fight for his life at some point, if not many times. A BX world allows for the possibility of a dragon flying over your head as you travel the King's Road or for a hill giant to be walking down a city street! This is a fantasy world.
That being said, most of these creatures are monsters, not NPC or PC "playable" races (BtB at least). But "monster' does not always equal "enemy." A decent reaction roll and the appropriate language slot can result in parley or even friendly communication.
Combat can be de-emphasized and other aspects of play can be focussed upon, but by default at least the threat of violence is deeply ingrained into a D&D setting. What can give this a more "BX feel?" Well, BX is a fairly lethal flavor of D&D. PCs tend to be fragile with their lower hit dice and -by the book- 0 hit points being dead. Even mid to high level PCs can be killed fairly easily, and morale rolls can lead to the better part of valor being exercised by monster and hireling alike. Compare that to some of the later versions of the game and you can easily picture a world where life can be a bit cheap and those that live by the sword are likely to pick their battles carefully as well as try to squeeze every advantage out of a situation. And once the battle is over, they will be sure to get as much of the spoils as possible to offset the risks.
XP for GP. That brief statement tells me this is a world where its inhabitants gain influence and become more competent by getting as much as they can for as little risk as possible. It's not the slaying of the monster, it's the treasure it was guarding. A BX world is a place where foul humanoids have piles of loot stolen from victims or looted from old castles they now infest. Half-rotted coin pouches lie among the bones in the lairs of terrifying trolls and gigantic spiders. And that axe of antique design wielded by the bugbear chieftain? It has a +2 enchantment on it.
This is a world of coin-filled coffers and magic swords. Of scrolls containing mystic spells or treasure maps. Of idols with a single ruby eye the size of a golf ball. Of dragon hoards, staves of power and magical rings. The DM may not wish to flood his world with magic items but in a BX world, such things exist and, even leaving it to the random treasure charts, the PCs will encounter at least some of them.
Assuming the characters live long enough, it's also a world where lowly murder-hobos and would-be heroes might accrue enough wealth, fame, and connections to become lords (and ladies) of the land themselves. It's not a place where everything is 100% fixed sociopolitically. Maybe there are wars, or dynastic struggles, or rebellions and invasions. Maybe there are young nations that are still growing. The point is even if your PC started as a turnip farmer, he could one day be a knight in a keep with a fiefdom of his own to rule.
What does all this mean? Have I answered the question? Well, no. probably not. But I don't think that it's a question that can be answered definitively. What I do think I've accomplished is to work through some concepts of what I think a setting should or shouldn't have to be a good fit for Moldvay/Cook.
And maybe it's done a bit that for you, too.