One of the things I miss about the earliest days of the TSR adventure modules is the fact that, while they were designed to fit into a particular campaign world (the World of Greyhawk Fantasy Setting), it was also completely possible (and it seems deliberately so) to plug them into one’s own home-brew setting with little or no conversion.
Take, for example, G1-3/D1-3/Q1, the famous “Against the Giants” modules. Although they were set in a particular section of the Flanaess, the basic plot – giants are raiding human settlements and need to be stopped, and some unknown force is the impetus for the raids – can be transferred into just about any campaign world. Modules like S1 “Tomb of Horrors”, while possessing some background information about ancient history and a particularly noteworthy villain of yore, can simply be plunked down into any campaign with a minimum of fuss (bear in mind that the module itself gives several different possible locations for the Tomb; it could be in Iuz, on an island in the Nyr Dyv, in the Bright Desert, in western Geoff, in an island off in the Solnor Ocean, or in the Vast Swamp, which seems to have become the default (and later canonical) locale.
Even modules that seem to be fully integrated with the campaign like T1-4 “Temple of Elemental Evil”, with its complex background of politics, religion, and history, can be placed in just about any campaign with only a small amount of conversion (or no conversion at all, if one fancied putting deities like St. Cuthbert and Iuz into their home campaigns– I know several of my early homebrew efforts had them as gods).
Contrast this with many of the modules that came later, especially the “adventure path” type adventures. Because they were tied to tightly into the background and ongoing story of a particular campaign world, it made it much more difficult to drop them into one’s own homebrew world. Contemplate putting something like DL2 “Dragons of Flame”, or “Cormyr: Tearing of the Weave” into a different campaign milieu. It’s perhaps not impossible, but it does present a much more daunting prospect.
Note this is not a function of Greyhawk adventures being more quasi-generic; many of the later titles were just as tightly woven into the fabric of the setting as any Dark Sun or Forgotten Realms module could ever aspire to. It’s a function of time, and as far as I can tell the switchover started in the early-mid 1980’s, when homebrew campaigns began to decline in favor of pre-printed settings.
This sort of portability is something modern designers might want to bear in mind when designing new adventures. Many OSR titles today are, in some ways, too generic. I think my favorite type is a mix; a module that is set in a particular setting, but which can easily be ported into other campaigns with little effort. That, to me, gives the optimal mix of background texture and portability that makes a module most useful.