Did you come up with rationalizations for the axes you had?
Sort of. A big part of it was that the "elemental" planes actually weren't quite so elemental. In D&D, the plane of Air is just that, Air. No land, just everyone flying around. In mine, the six Outer planes are all just variations of Earth that lean in one direction. For instance, the "Plane of Fire" was actually just a desert world. Specifically, it was called Arrakis, so you can guess some of its key elements. Likewise, the "Plane of Water" is a world where no land was above the water's surface (and no, Kevin Costner had nothing to do with it); floating cities were built on the continental shelves, with the largest city being Atlantis. The Plane of Earth is one where the surface was uninhabitable, so civilization developed in caverns underground (i.e., Underdark), while the Plane of Air... well, ever see the anime Nausicaa? All the lowlands are filled with volcanic/toxic gases, so the only life is the stuff that can fly or perch on mountaintops.
So the difference between the Plane of Life and the Plane of Death is simply environmental; on Life, there's plenty of sunlight and a moderate amount of the other elements, so life flourishes, and the plants are all photosynthetic. On Death, it's permanently clouded over (which doesn't automatically mean an Ice Age, but it's close), so most of the life forms are parasitic, have vampiric traits (like the Shades) or can form protective shells (like the Gargoyles), and it's a lot harder for stuff to survive. With that in mind, connecting them by the Plane of Light makes sense, since they're sort of like what'd happen to Earth at the two extremes.
Thinking of it as extremes helps with the others, but it's still mostly rationalization. If you think of Force as the intensity of movement and Nexus as the unpredictability of the direction (which works better if you think of it as Chaos), then it makes some sense; each element is the extreme of one of the two variables and moderate on the other.
Earth only moves rarely, but when it does, it's in moderately unpredictable ways (earthquakes, rockslides).
Air moves quickly, and while it's somewhat variable, it still mostly blows in one direction on a given day.
Water moves somewhat slowly, but it flows in predictable ways in one direction. Rivers don't suddenly reverse direction.
Fire also moves somewhat slowly, but it expands chaotically, in random directions. Fires can expand upwind, jump across streams, etc.
So, the difference between Earth and Air is a matter of law/Force, while the difference between Water and Fire is chaos/Nexus. Remember, this doesn't have to make sense, scientifically; it's the metaphysics of a primitive world, given life. Besides, from a game sense it's more balanced that way. Each elemental triad has a wide variety of abilities, but doesn't cover everything.
Also, the reasons I did the planes those ways was that in this campaign, planar travel spells are MUCH easier to get; the basic self-only planar travel spell is a level 2 spell, and the simplest group version was level 3 or 4. So, the campaign jumped around constantly.
