If you don't mind, I'd like to occasionally bang on about it, starting today.
4th Edition D&D has what the developers call an 'implied' setting, a selection of people, places, and histories which are assumed to underly most games running in the 4E rules. The Gods fought the Primordials, Orcus is the Demon Prince of Undead, the Feywild and the Shadowfell exist alongside and are coterminous with the Mortal World... and so-on. Thematically, the rules also talk about a presumed 'Points of Light' (PoL) setting, a world where small pockets of civilization are continually at threat from the shadows of the larger World. All pretty good stuff, rich pickings for the game, and well-presented both in the core and subsequent expansions.
Many DM's won't care about the implied setting and will completely ignore it, forging new worlds with their own geography, Gods, origin stories, cosmology and all of that stuff. Others might not have the inclination or the time to engage in such world-building antics, and instead accept the implied setting for what it is, using the Nentyr Veil straight out of the pages of the DMG, and running published adventures almost verbatim. Most though, including your humble host, fall somewhere in between, so what does the implied setting mean to them... or more to the point, what does it mean to me?
When it came to building Cradle Plain, I didn't have the time to design and write everything from scratch, much as I'd have liked to, so I already knew I'd have to pull a lot of stuff from the various adventures and supplements published by Wizards. I also knew that a weekly game, which was something I hadn't had the luxury of for a long, long time, would afford me the delicious opportunity to forge a campaign with multiple stories and long-running characters which could grow directly from the actions of the PC's. We play four hours a week, which means, technically speaking, I only have to actually prepare four hours ahead of time; in practice, I prepare far in advance of that, but the further out I go, the more purposefully vague I force myself to get, in order to accomodate several PC's whose actions I couldn't possibly predict that far ahead of time. Although I would never go so far as to describe myself as a sandbox DM (I love my metaplots way too much for that), I did want them to feel invested in the world and to feel that they were driving the action.
So there I was, with the opportunity to build a dynamic campaign, but with the necessity to use pre-published material... books which would have the implied setting fully present and correct. I panicked briefly, then set to work.
I liked the Points of Light theme for conjuring a feel which wasn't at odds with my D&D experience; you could still have fantastic cities with mile-high towers, aloof mages and power-hungry Kings (the points of light, i.e. your cake) alongside the unforgiving, dangerous wilderness and the cold terror of the dungeon (the darkness in between, i.e. eating of said cake). So straight off I saw no reason not to import PoL lock-stock-and-barrel into the campaign.
Next, geography. I wanted a world a couple thousand miles across, a few weeks of hard travel end-to-end. This would give me enough space to brush in the Points of Light theme without being so big that the PC's were effectively locked into a small corner until the inevitable introduction of Paragon-level teleportation and overland flight rituals. I wanted them to be able spread their wings, travelling the land using good old shoe-leather first, so that when more convenient magic finally came along, they could appreciate it all the more. So this means we're in an isolated tract of fertile plain whose edge touches the sea, or as the campaign wiki puts it:
Shielded by the thick Midron Mountains to the north, sheltered by the branches of the massive Arubisath forests to the west, and harboured by the turbulent seas of the Greyland Wash to the south and east, Cradle Plain has existed as a somewhat independent confederation of lands and peoples, distinct from the massive turmoil which has repeatedly overtaken more distant regions of the world, or Outside, as it is known to the locals.
The campaign is enclosed, which in practical terms gives me some necessary control over where the characters are able to roam, but equally importantly, reinforces a feeling of isolation from the rest of the World. It has a good variety of terrain, magic coming to the rescue where semi-realistic climate patterns do not, several large ports of call for the adventurers to enjoy the feel of the area and pick up quests, and, I hope, enough intrigue to tantalise them into travelling far and wide without necessarily waiting for me to prod them in the right direction.
I knew I wanted to build the campaign around an earlier idea revolving around Citadels of power which were believed to mark the final resting places of celestial beings. This synergised immediately with the war between the Gods and the Primordials, and although that isn't spelled out in the common histories, it is assumed by Plain scholars that the Citadels are some kind of relic of that conflict. In any case, the imprecise history of the Citadels, what they are and what they're for, is one of the central themes of the campaign. The Primordials themselves are curiously absent from the accepted histories, consigned to a time known as 'Beyond Reckoning'. It's safe to say that, as a force even gods have to be fearful of, they, er, might well pop up in the future, but it was still a fortuitous and satisfyingly good fit for an idea that had been floating around in my head for the better part of a decade.
Pantheon-wise, for a long time now I have tired of building gods and portfolios, and normally don't apply much thought to this at all. I had also grown tired of the 3E divine healing paradigm and the classes that were intended to do it. In fact, the original 3E Cradle Plain idea was that there were no gods at all, with a lot of divine magic having been subsumed into the arcane portfolio, and healing magic itself transmogrified into a radically up-shifted version of alchemy or other 'technological' alternatives. I liked it at the time, but this was to be a campaign built off the launch of a new edition, and I didn't think it would have been appropriate to rip whole portions of the game out before I knew anything about how it ran. Shame really, since the new concept of Power Sources would have made it thematically quite easy, and the proliferation of non-magical healing throughout the game would also have drastically lessened the impact.
Besides all of that, Healing Surges actually make healing much less irritating these days; no longer do clerical healbots have to accompany the party and sigh as they substitute yet another utility for a Cure spell. Instead, Leader classes get to throw themselves right into the action, sometimes doing damage and healing their pals at the same time (the very idea!)... and you can get by reasonably well without a healer at all, if you need to (alright, you could do that in 3E with a curing wand of some kind, but that was crap).
So given that the ground-up change to D&D healing that I thought had become necessary in 3.5 simply wasn't required in 4E, it turned out I could relax a bit about the pantheon as well. Still, I didn't want to restrict everyone to the gods laid out in the implied setting, so the campaign wiki remained mostly quiet on the subject of which deities existed, other than that a few of them seemed to have been cast down during a time known as the Fall (and the Citadels that now stand at their final resting places carry their names: Emerandes, Phalax, Arranea). This gave my players the freedom to invent their own deities, and we could worry about who they are, and where they existed in the objective truth of the campaign universe, if, and when it became relevant. Are they aspects, or even Exarchs, of more 'famous' gods, or entirely separate entities, electing to show themselves only to a chosen few? Who knows, and it might never even matter to anyone except the PC that reveres them.
Next time: Cosmology, history, and all that good stuff.