Dual Natures of Outer Planes

This only applies to Avalas, the first layer. Avalas is warfare, Thuldanin is the ruins of warfare, Tintibulus is magic (or even emptiness, when not even ruins are left), and Ocanthus is blades. The theme that ties them together is Conformity.

The Beastlands' theme is animals. Wilderness is just a side-effect of this.

Cooperation, community, industriousness, symbiosis. "Industiousness" is the theme that Planes of Conflict emphasizes, though I think Cooperation is better for the plane - the union of two unlike qualities for the greater good.

Treachery. It is a plane of imprisonment (as is Pandemonium, Ravenloft, and the Plane of Ooze), but the imprisonment is caused/sustained by the treacherous nature of its prisoners.

Exploitation.

No, it's not a plane of dying, but it is a plane of the dead. The first layer, Oinos, is primarily about disease, the second layer is mists/cold, and the third layer, perhaps shades. Each layer has realms of the dead, so I suppose you could claim that "The Dead" was the theme, but each layer is much more than that. The theme that ties the plane together is Despair.

Pretty much just clockwork.

If any outer plane has "mundanity" as a theme, it doesn't belong as part of Planescape. Fortunately, there's nothing mundane about the Outlands - the home of gods and spirits, with mountains vaster than entire Prime worlds (including an infinite spire), oceans deeper, plains vaster, deserts dryer than anything known on the Prime.

As with Acheron, "warfare" is only a theme on this plane's first layer. The second layer, Muspelheim, has the theme of Fire. The third layer, Nidavellir, is about darkness/artisanship. The theme that ties them together is Creativity/Inspiration or perhaps Heroism.

As for the others:

Mount Celestia: Virtue. The entire plane is built around the Virtues, and you need to better comprehend the Virtues in order to rise from one layer to the next.

Elysium: Self-Knowledge/Understanding.

Beastlands: Dreams/Archetypes

Arborea: Passion/Stories/Memory

Ysgard: Inspiration/Beginnings/Endings/Fire and Ice and Earth

Limbo: "Will" isn't bad, but it's only one aspect of the plane. Change/metamorphosis is probably just as important.

Pandemonium: Madness/Darkness/Wind

Abyss: Atrocity. Each layer is more atrocious than the last.

Carceri: Betrayal

Hades: Despair or, more obviously, Grayness

Gehenna: Exploitation or, more obviously, Volcanoes

Baator: Political Corruption

Acheron: Conformity/War

Mechanus: Clockwork/Mathematics

Arcadia: Harmony

Outlands: Balance/Différance/the Tao/blending/juxtaposition

Honestly, i'd like to see someone sit down sometime and put together netbooks for each plane. Exploring these sort of themes, the characters and personas that exemplify it - and modules for it....

A netbook is something like the PSCS that we have here.

It's a online only book - usually PDF, sometimes doc or rtf and the reaaaallly old ones are just plain old text files.

I like most of the double torus view of the Planes. In this model, Elysium goes with Life or Benevolence towards Life (which explains the otherwise puzzling guardinals.) The Beastlands take this and add more chaotic influence. (Also, the rest of the Benevolent Planes fit mostly humanoid beliefs about paradise. The Beastlands help remedy this imbalance.)

The Gray Waste goes with Death, but especially Death of the Spirit. We might throw in deception, since everyone living on the sodding plane pretends to have color in their appearance. But I'll have more on that when I'm awake.

While I agree that the planes shouldn’t be reduce to a few words for its description or to limit their themes; I do find that these broad categories actually help me to flesh out new ideas much more than being told “It is a plane of Lawful Neutral to Lawful Goodness”. It should inspire ideas not limit one's choices.
Saying that, I’ve modified the planes quite a bit (including adding ten more layers - which I won't get into here) so some of these may not apply perfectly to the canonical model, but here are the themes I use:

*Abyss: Destruction, anhilation
In my mind demons seek to destroy the creations of the gods whereas devils seek to usurp the control of the existing order (as they see themselves as superior beings). Some demons are just engines of destructions while others destroy things in more subtle ways (e.g. destroying a person’s faith, destroying the love in a person’s marriage, etc.)

*Acheron: Pointless conflict, the futility of war.
I have those that die in conflict here rise from the dead the next day and be forced to join the nearest military unit regardless of one’s personal interest. Your foe one day becomes your fellow soldier the next. After a while, the futility sets in but you HAVE to keep on fighting

*Arborea: Passion, living life with gusto and bravado
My version is more Greek myth and less elfin

*Arcadia: Seeking the best means to govern
I took my lead from the sugeestions on the Planar Renovation Project, which I think makes the plane much more interesting place to visit – here different groups compete to determine which one has the best model to create a Utopia

*Baator: Tyrrany, controlling others
See my distinction regarding devils and demons under “The Abyss”

*Beastlands: Nature, the ideal of the “noble savage” who finds “civilization” to be corrupting to one’s soul and which isolates you from nature

*Bytopia: Invention, creativity, ingenuity
Again see some suggestions under the Planar Rennovation Project

*Carceri: Imprisonment, bitter resentment (which leads to betrayals)
I incorporated a lot of Kafka-esque themes and the paranoia aspect of Bentham’s Panopticon prison design into my personal re-design of this plane

*Elysium: Life, healing
I make this a plane where miraculous remedies can be attained, if one can demonstrate one’s goodness. But I also added some areas with odd results: e.g. an area where fertility is so powerful a force, that men can get pregnant; an area where healing occurs but often with a beneficial (but unsightly) mutation (like a drowned man who is brought back to life with gills)

*Gehenna: Bullying and intimidation
I pictured this as a plane with various Mafia-type organizations and street gangs all trying to gain more power over the other. It doesn’t usually result in conflict like the Blood War but it is very uncomfortable to be a weak person being pressured, intimidated and manipulated by these groups

*Grey Wastes: Death, emotional death

*Limbo: Chaos, change

*Mechanus: Law, regularity and routine

*Mount Celestia: Virtue, striving to improve oneself

*Outlands: Balance
Admittedly, I haven’t found a good use for this plane except as a starting-point or middle-ground to the rest of the planes. I’d love to see the Planar Renovation Project tackle this one

*Pandemonium: Madness, hidden secrets
I added that while it causes madness, sometimes it also grants deep insights or secrets to the multiverse. Most people still wouldn’t risk it; but sometimes people can pick up secrets from the babbling of the mad

*Ysgard: Heroic glory
I had this differ from the heroics of Arborea/Olympus by having the petitioners here embrace the Norse idea that the brave forge their own destinies (comparable to the philosopher of the Fated). The Greek petitioners perform heroics for the glory of the tale that will be told. The Norse petitioners perform heroics so that their own personal glory that people would remember