Like many Planescape fans, I was drawn to the Big Picture it provided for looking at the D&D multiverse. Its planar symmetry implies a kind of grand order behind all the bizarre places and creatures roaming the game’s pages.
Here’s the thing though: the implication of order is only skin-deep. The planar symmetry doesn’t make much sense at all, once you think about it. Here are a few examples:
Air opposing earth makes a certain sense, but fire opposing water? C’mon! One’s a chemical reaction, the other is a state of matter. And what’s with the quasi-elemental planes? Negative energy annihilates air completely, extracts the salt from water, and splits earth into dust. The effects of positive energy are also inconsistent.
The creatures of the planes don’t make much sense either. It’s tempting to imagine underlying patterns in the fact that each outer plane has its exemplars, but again, the symmetry is only skin-deep. The tanar’ri of the Abyss come in all shapes and sizes, as makes sense for a group spawned by a chaotic plane. But the slaad of Limbo are all anthropomorphic toads, and the eladri of Arborea are a race of super-elves. Meanwhile, the archons of Mount Celestia are almost as varied as the tanar’ri.
It makes sense so long as you don’t think about it.