Even though I agree that Gloranth attempts to portray a pre-modern world, I wonder how much of a benefit post-modernism was. Post-modernism was in its ascendancy in 1966 - when Glorantha was started. Tolkien didn't have access to those ideas when he was developing Middle Earth. The best he would have had access to were the ideas of Marx and Marxism. Although the former of those, Marx, is superior to post-modernism, Tolkien obviously never got that far, as it would have been awash with Marxism (something even worse than post-modernism.)Consider Tolkien. Middle Earth is a world which has an absolute truth. Eru created the world, and those who live in accordance with the “mind of Eru” are good while those who go against it are bad. Goodness, truth, and righteousness are the rewards of those who side with Eru and the Valar. Those who defy Eru, from Melkor and Sauron right down to the Easterlings, fall into error and ultimately suffer. This is the kind of absolutism offered by Christianity, which is not surprising considering Tolkien's own devout Catholicism.On the other hand, we have Howard. Howard's Hyborian Age has no absolutes, no good, no truth, and no real evil (its demons may be alien and inhuman, but don't qualify as evil the way Melkor does, because there is no absolute good to be the opposite of). The Hyborian Age is an almost Nietzschean paradigm where strength is the only real virtue.Michael Moorcock offers a very different paradigm. His work seems to say that any absolute—in his case absolute Law or absolute Chaos—is intrinsically unbearable and that the only wholesome route lies through balance.With this in mind, let's consider Glorantha. If Middle Earth embraces a single truth, Hyboria mocks truth, and Moorcock's Million Spheres seek a balance between truths, Glorantha says to us that truth is in the eye of the beholder. Truth exists, and can be obtained, but it is a cultural and—to an extent—personal truth not valid for everyone. Truth is a local, rather than a universal, phenomenon. For example, most cultures in Glorantha agree that there was a time when the sun disappeared from the sky. The Orlanthi say that the sun was a tyrannical emperor, and that mighty Orlanth slew him to liberate the cosmos. However, the sun-worshiping Dara Happans say Orlanth merely slew the solar emperor's son(the divine sun himself was far too great to slay), and that the solar emperor died of grief. Now, in any other world, we might just say that these too cultures have different beliefs and leave it at that. But in Glorantha, an objective third party—like, say, a God Learner—could go to Dara Happa, leave the mortal plane, and personally witness Orlanth slaying the solar emperor's son. The same God Learner could then go to an Orlanthi holy site, enter the Hero Plane, and personally witness Orlanth slaying the tyrannical solar emperor himself. In fact, he could get powers from participating in two contradictory myths!Because of this, Glorantha embraces a pluralism unprecedented in other fantasy settings. Tolkien is culturally pluralistic, but his world operates around a single truth. Hyboria is also culturally pluralistic, but truth is ambiguous at best. And Moorcock may have a Million Spheres, but all are governed by the same struggle. Even Dungeons & Dragons, with its “everything but the kitchen sink” approach to setting design, still has the cosmic absolutes of law, chaos, good, and evil (lawful good is lawful good, from world to world and setting to setting). Glorantha is wholly relativistic.This pluralism is not the result of a modern, politically correct, “accept all faiths” viewpoint, but rather indicative of the pagan attitude, which is wholly consistent with the mythic, bronze-age world Glorantha portrays. When we examine the religious attitudes of ancient Mediterranean and Near Eastern cultures, for example, we find that they are perfectly aware of foreign gods, and accept their existence, but view their own deities as being more central to their lives. A clear example can be found in the Ten Commandments of Hebrew scriptures, where Yahweh tells his people “I am your God, and you shall have no other gods before me.” Note he does not say, “I am the only true God, and all other gods are false.” This attitude did not appear until late antiquity, a period which falls long after Glorantha's scope (except, perhaps, where the Malkioni God-Learners are concerned).
It's interesting that besides Glorantha, I know of no other game-world that embraces plurality. Most are modernist (Forgotten Realms, Dragonlance, Greyhawk, Traveller universe, WoW, Midnight, etc.) You really have to go to literature before you start to get anything interesting (Le Guin, Leiber and the ones mentioned above).