[Edit: this started to ramble a bit, so I decided to cut myself off and save the rest for a later post.]
While there is certainly no shortage of settings for fantasy RPGs, it seems that some are better suited to certain games than others. I’m not interested in dissecting every published setting out there. Rather, I’ve found myself mulling over those aspects of some fantasy worlds (whether it’s a game setting or a piece of fiction) that seems -merely in my own personal opinion and experience- to embody some facet or facets which fit a Basic/Expert D&D game.
To begin with, I find BX and OD&D lend themselves a little more towards "swords & sorcery" and little less to "high fantasy." I'm not sure I have hard and fast definitions for either of those terms, but let's see if we can't parse that a little bit.
Wikipedia defines the swords & sorcery genre as:
"... a subgenre of fantasy generally characterized by sword-wielding heroes engaged in exciting and violent conflicts. An element of romance is often present, as is an element of magic and the supernatural. Unlike works of high fantasy, the tales, though dramatic, focus mainly on personal battles rather than world-endangering matters. Sword and sorcery commonly overlaps with heroic fantasy."
And high fantasy as:
"High fantasy is defined as fantasy set in an alternative, fictional ("secondary") world, rather than 'the real', or 'primary' world. The secondary world is usually internally consistent, but its rules differ from those of the primary world."
The theme of good vs. evil features heavily in such stories as well. Whereas S&S tends to have more personal battles. But I don't think of Moldvay/Cook or Labyrinth Lord quite as "Swords & Sorcery" games. They are after all, still Dungeons & Dragons; and while the roots of D&D may be more John Carter than Aragorn, it's not quite pure Conan to me either. The truth of it (if there is one) seems to lie somewhere in between. Which brings me to alignment.
3 v. 9
Classic D&D uses the three-point alignment spectrum as opposed to the nine of AD&D. There is no 'Good' or 'Evil'. Just law, chaos and neutrality. I have heard the opinion that this is a simplification for a "basic" game, but I don't find that to be the case. Rather the opposite really. I've rambled about this in the past, so I won't rehash it here. But I do feel that keeping the three point system and what it entails firmly in mind influences the nature of the setting. One need look no further than the introductory text of the seminal B2: Keep on the Borderlands for an indication of how Gary envisioned Chaos in the world:
"The Realm of mankind is narrow and constricted. Always the forces of Chaos press upon its borders, seeking to enslave its populace, rape its riches, and steal its treasures. If it were not for a stout few, many in the Realm would indeed fall prey to the evil which surrounds them."
One could read this as a classic "good vs. evil" set up, and -to be fair- that's perfectly valid. But reading the wording closely, I notice two things: Firstly, it's the "Realm of mankind." A human nation. This realm is "Narrow and constricted," seeming to imply that civilization has a limited reach. Much of the world is wild and quite possibly chaotic.
Second, while the text does talk about "evil" preying upon the populace, the fact that this evil seems to be interested in personal gain (slaves, riches, etc.) and not necessarily covering the world in darkness makes this less about EVIL and more about Us vs. Them. Sure, the "them" in this case are primarily humanoid monsters, as opposed to just some other country of people, but this IS a fantasy game after all.
It makes sense to me that the majority of people you'd meet would fall under Neutrality in this model. Most people have a natural desire to just get along and live their life. They recognize the utility of law & order, but they also don't want to jettison their desire for a level of personal freedom. Benign self-interest is the rule of the day. I'm reminded of a quote from The Hobbit about the dwarves when Bilbo goes through the hidden door.
“Dwarves are not heroes, but a calculating folk with a great idea of the value of money; some are tricky and treacherous and pretty bad lots; some are not but are decent enough people like Thorin and Company, if you don't expect too much.”
That last part pretty much sums up my idea of neutrality. So in a world where that's where most people fit, it's pretty reasonable to expect their motivations to be more personal and self-interested. As opposed to altruistic world-saving. It also follows that players are more likely to accept the hooks that benefit their characters directly, as well as trusting NPCs who seem motivated by self-interest as well. Adventurers tend to be a greedy bunch, as a rule. Especially in a game where advancement is largely achieved through the finding and gaining of wealth. At least, I've also found that a better fit in my BX games.