The campaign world, Edhel, was once known as Erdelane. In mythical times, it was sundered, cleft in two parts, which are now known as Edhel and Rhane. The silent 'h' in the names are subtle reminders of the hollow nature of both half-worlds.
Anyway, the world and its moon are twin planets, of equal size. Both have verdant continents, blue oceans, and white clouds.
As a result, the moon is much larger in the sky than in the real world. Tides are more impressive as well. Moonlight is blueish rather than silvery.
The Ethereal plane is replaced by several ethereal layers, one being close to the material plane, and then each other being farther and farther.
Each planet has its own ethereal layers. The Astral plane has one ethereal layer as well.
While a planet's material aspect is finite in size, its ethereal layers aren't. They are infinite and definitely non-euclidian. While every point in the material plane has a corresponding point in the closest Ethereal layer, the reverse is not true.
There are no outer planes. Instead, there are ethereal realms.
The gods worshipped by surface races live on the moon. As a result, clerics of all religion watch the moon at night, looking at the divine cities through telescopes. The day is for mortal activities (like mundane work) and the night for religious activities (like prayers).
There are no inner planes. They are, on the other hand, primordial dimensions. When you are in the dimension of fire, you do not move to another plane. Instead, you replace your perceptions of the three physical dimensions of space (you know, heigth, depth, width...) by three metaphysical dimensions of fire.
In effect, you see only one subset of matter and energy. In a way, it's like a 2D sectional view of a 3D object; except it's a 3D sectional view of a nD universe.
The geography of the dimension of fire does not mimick that of the material plane, since they do not use the same spacial "coordinate system." You can find one (or more) primordial plane for any energy or matter you can conceive, you just need to find three proper dimensions. That way, you can have a plane of vapor, or of sound, or of ligthning... You can even have a plane for a color, which will be made only of all things in the universe that are of that color.
Divine magic is separated into theurgy (god-based) and tellury (nature-based).
Everything in the universe is sustained by an energy known as leï. This leï exists in at least four different known phases. Depending on which phase it is in, leï is positive energy, negative energy, magical energy (the one you use, notably, to cast spells), or raw energy (a.k.a. force effect). So, while it is easier to heal someone by infusing him with more positive leï, it is also possible to "transmute" his negative energy into positive one, forcing part of his leï to change phases.
Also, it's the reason arcane and psionic spellcasters tend to have weak HD. They transmute their own life energy into spell slots.