I recently had suggested to me by a gamer I know, the article "What not to include" by Arthur Collins in the February 1992 issue of Dragon Magazine. I don't have Dragons going back that far, but a quick Google search found me the text, and I was able to read it. The subject--and the discussion of it specifically from that article--brought to mind a lot of FRPG type topics, so I decided to make an FRPG post out of it.
First of all, the perspective of the article is definitely one of a D&D gamer playing a D&D game. It's talking about the notion of defining your campaign settings as much by what they don't include by what they do. Less is more, in other words. In an FRPG that's not D&D, the situation isn't quite so clear-cut, since few of them have as many materials as D&D does to navigate through. That said, there's some great generic points to be made about designing a fantasy setting, whether for use with D&D rules or not, which doesn't feel like your typical "throw everything in" D&D setting.
And before I begin properly, let me also make a quick aside. The EBERRON campaign setting is often (falsely) accused of being such a D&D kitchen sink, which incorporates every single D&D element ever. This isn't really true. Eberron is designed such that if there's an element within D&D that you want to include, you can find a place where it makes sense to include it. That does not mean, however, that the element is assumed to be there by default. I think Eberron is notable amongst D&D settings (although not unique) for having a rather strong twist or thematic focus that differs from "generic" D&D--yet even so, it also suffers a bit from having to be so D&Dish when I think it really is reaching towards being something else quite frequently.
Anyway, neither here nor there as my own setting DARK•HERITAGE is certainly not really a D&D setting, and I don't recommend using D&D rules to play it. Nor is the focus of this series of posts with the FRPG tag focused on using D&D rules either. So, I'm going to go through some of the main points in the article, where relevant, and talk about them in another FRPG environment.
Related to that; a lot of just plain animals can be pretty exciting, and not every encounter needs to be a combat encounter. While you may not need to get all violent slaying the ravenous raccoons who sneak into your camp at night to eat your rations, it could still add an interesting challenge to the game. And a pack of wolves or a pride of lions or herd of over-protective mother elephants should be a touch-and-go situation for any actual person to have to deal with.
3) Religions. I've talked about this a bit before; religions in D&D are a bit difficult because of the cleric class (although more recent iterations of the game have tried to decouple the cleric from faith in the divine per se) but in a non-D&D environment, religions can play all kinds of interesting roles. Religions can compete, of course, or they can be subject to a great deal of syncretism (such as the religion of the Romans, which seemed to absorb cults and gods more often than not--Isus, Sol Invictus and Mithras, as well as the adoption of numerous Greek aspects being the most obvious--although cults of Dionysis, Judaism, Christianity and druidism were seen as divisive alien cults that were viewed with suspicion if not outright hostility--quite famously.) Religions in D&D are often very monolithic, but in history, such was not the case. The holy wars that racked Europe such as the Thirty Years War, the English Civil War (and later Glorious Revolution), the Avignon Papacy, etc.--there's plenty of drama to be had even within the bounds of a single or closely related confessional traditions.
That said, I think polytheism and fantasy just "go together" quite well. Fantasy that too closely resembles the middle ages, and has either Christianity or a transparent analog to it, just makes me feel a bit uncomfortable. It's a little too close to social commentary, or something, and usually not in a good way.
Its also possible, of course, to do away with religion entirely--just make it more or less fade so far into the background that it's a non-issue--especially in an FRPG system that doesn't have clerics.
4) The Multiverse. "Planes" of existance is really a D&D term. I've never heard anyone else talk about fantasy realms, or other dimensions, or what have you as "planes." It appears Gygax borrowed the term from theosophy--although maybe he got that himself from Lovecraftiana (Lovecraft notoriously loved to refer obliquely to theosophists as being "clued in" to his Mythos, at least somewhat.) Despite the label, however, most D&Dish planes have at their roots either classical or Norse mythology, or Judeo-Christian cosmology, or something similar. Because of that, although somewhat specific to D&D and esoteric, they're also pretty familiar to fantasy fans already from their more classical sources.
That said, give some thought to a) what exactly "the planes" or "outer realms" or "other worlds" or whatever you want to call them actually are rather than simply go with the familiar, and b) give some thought to whether or not they actually have any impact on your setting other than mythologically. Although going directly into Hell, or crossing the River Styx, Asgard, or whatever may be something that happens on occasion in the mythological and literary source material, it's not really very frequent in fantasy, and it may not be relevant to you. And if it's not relevant, I always remember Ray Winninger's First Rule of Dungeoncraft--don't create anything you don't need.