On the basis that somebody has to do it, here it is. How one may apply bits of David Eddings to fantasy RPGs in interesting ways. The impetus to do so come from this article. (Really, do read this first, comments and all).
I am going to work on the Belgariad and the Mallorean, to limit the scope of things somewhat.
Is this meant to make you run out and read these books? By no means. But if it is part of the canon of fantasy literature, why not mine from this seam for one's RPGs?
0) The Rivan Codex
Why is the Zeroth point? Because it's cheating, if you will.
The Rivan Codex, in this context, is the name of a book of David Eddings's notes, world building, in-universe literature for the Belgariad and Mallorean - and his advice to authors. If you've ever thought Eddings was formulaic, this is where you find his formula (quite literally).
Either way, this is a source for world-building. Currency, dress, manners, forms of address can all be found here (whether or not they made their way to the books). In-universe religious scriptures are also available. If you needed to bring into being a society in-universe rapidly, this would be a decent crib with which to do so.
It was available as a paperback in Britain about a decade ago, so I shouldn't imagine it will be too difficult to find if you want it.
1) The alignment of deities to Peoples
The deities of the Belgariad and Mallorean are all aligned to a given nation. Fantasy RPGs all seem to rather opt for a form of monotheism on a somewhat Abrahamic model or something Hellenic - or some combination thereof. Either way, worship is available to all rather than having some measure of exclusion or cultural boundary about it. Perhaps so that the Half-Orc Chaotic Good Cleric of Kypris can be on speaking terms with the Drow Neutral Evil Cleric of Scylla.
However, this alignment offers a certain chthonic or archaic flavour of theology to proceedings. It tones down the interlinked world of trade and metropolises in favour of isolation, distance and pockets of unthought strangeness.
2) Caste and divisions of Peoples
Of these nations, affiliated to deities or otherwise, a number are divided up from a common origin. Several on the side of good derive from the sons of a hero-patriarch ancestor. Those on the side of evil, from the denizens of a city, exiled by their mad god.
National characteristics or castes aren't necessarily an obsession of the books in question, but they appear nonetheless. It could offer a certain historical-cultural depth or sense of time to a setting to have inhabitants have this to refer back to - and it isn't quite something I've seen elsewhere.
3) Merchants and Drasnia
Merchants keep playing a part in these tales; I won't say they are central, but Eddings tried in the time in which he was writing to play up the roles of a trade that he thought hadn't been displayed by the genre at that point. Perhaps things have changed. However, this still applies to the world you may be building - is it just made up of wizards and warriors? If so, is there a good reason?
Eddings's resident mercantile nation is thankfully not terribly reminiscent of Venice. Indeed, it is barely even sea-faring. Drasnia is northerly, cold and largely land-locked. It is famed for heavy infantry in battle. Early notes portray it as herding reindeer. A semi-Russian flavour hangs over elements of it, though not to the point of samovars, Soviets or tsars. The merchants of Drasnia all tie into the foreign intelligence departments of this kingdom, which seems a little heavy handed but can perhaps be forgiven. It is the easterly trade routes that sustain these merchants.
Drasnia seems like it should be quietly hooked onto other worlds as a form of contrast. I would characterise it as having a sort of quiet dynamism that contrasts with some rather manic depictions of mercantile bustle. Perhaps this appeals.
4) Demon-worshipping Tribes
There is an episode towards the end of the Belgariad in which our hero and two others must separate from the larger forces of justice and make their way across a dozen tundra. The tundra is inhabited by those tribes of races that were not adopted by any given deity. They have instead turned to summoning demons as protectors and totems of their particular tribe.
This calls out for a hexcrawl. A slow progress over difficult terrain, with hostile forces wielding unstable magics. The frozen ground dotted by totems and anti-demonic markings or trail signs. In the Belgariad, our heroes must rely on guile, despite their power. Perhaps this is an interesting mechanical usage: you might be level ten, but for the time being it will be most useful if you pretend to be level three. Fancy sword-play or potent magics will clearly demonstrate that you are a person of mickle might, and one to be watched - hence it will blow your cover.
5) Murgo gold
This makes an appearance in the early books of the Belgariad. The red gold of the Murgos (the most warlike of the nations serving the mad god Torak) is known for its quantity and its way of finding itself in hands of the corruptible - from whence it calls to its fellows and kindles greed for more in the heart of its owner.
Murgo gold, or something much like it, seems an excellent concept to employ in fantasy RPGs; a commonplace cursed treasure, detectable only by those in the know. What could be a more fitting thing to come out of a ruined, cursed place? A secondary, murky currency for the underworld or a burden for an adventurer - Murgo gold could be a useful mechanical addition to a game.
In closing this little missive, I should like to take a moment to note the quiet interest of dipping into The Rivan Codex and reading a little about Eddings himself. I have kept his books on my shelves, even where others have been removed. Even if this article, that which inspired it, and indeed my own view on his work comes from a place of relative detachment or even antipathy, I would not deny the man's success or that he was possessed of some skill as a teller of tales. David Eddings and his wife and long-time literary collaborator Leigh Eddings seem to have been the sort of All-American talented folk that appear in Neal Stephenson books. Part of me would like to know more about them - and, I suspect, to thank them.