Noted in your forum thread, but the best descriptions of Acheron and Carceri that I have seen—based in, but not directly quoted from, actual text—are those by Jade Ripley (who goes by Lord_Gareth here and numerous other places, including GiantITP.com’s forums):
Acheron
Acheron has no architects. None of the Outer Planes do. [...]
Yes, Acheron looks a little Chaotic on the outside. You know what, from the outside Carceri looks a little lawful. But Acheron isn't chaotic. Acheron is battle without resolution, law without harmony, order without structure, misery without hope, death without glory, unity without individuality. Acheron is not a plane that hates you; indeed, it is Acheron's utter indifference to you that eventually kills you. Acheron is the grinding monotony of hopelessness, and it is the weary horror of cynicism so great that it consumes morality. The sergeant who grows weary of fighting corruption and embraces bribery goes to Acheron; the office drone who takes out his misery on others by providing them barriers to actual help goes to Acheron. It is the punishment for which there was no crime, the penalty without a violation, the monolithic crushing indifference of Law with no moral compass, of conflict without belief, of tyranny without vanity.
Acheron doesn't hate you.
It wants you to die anyway.
Carceri
Carceri, the Red Prison, is a plane of injustice. Carceri is a mockery of law and lawful thinking, seen through a lens of powerful and self-defeating malevolence. Carceri is full of vain struggle, wretched anguish, trapped rage, and a heady mix of hope and despair that drives its inhabitants mad. From the outside, Carceri looks lawful - it is a prison, after all - but the thing that separates Carceri from ordinary prisons is that no one is in charge, and despite delusions to the contrary no one can ever be in charge. Though the denizens of Carceri make some effort to separate 'prisoners' from people who 'just live there' (that is, who are not locked up in a specific prison complex, prison structure, or torment) the truth is that all being in Carceri are prisoners, trapped there by their own fear, hate, mistrust, paranoia, vices, greed, desires, and malice.
And, really, that's the thrust of Carceri. The Red Prison mocks you with hope, offering escape and giving only frustration and despair. Even if you leave, it drags you right back, proving that any escape is merely temporary. Its "wardens" cannot leave, and they cannot impose order on their prisoners any further than the reach of their weapons. Riots and murder abound, a seething mass of hate and frustration that shakes the bars and rattles the cages, echoing through the plane and mixing with the shrieks of pain and pleasure. Really, it's not the plane keeping you there, it's you.
It was always you. It's Carceri's greatest and cruelest irony. In the end, the reason you will never leave is that even if you were fit for society once, you never will be again. You can leave Carceri, but Carceri, it never leaves you.
As for Gehenna, the landscape is rather uniform: a constant slope, a constant rain of fire and rock, and a constant absence of much of anything else. It is a barren landscape, and that uniformity is fairly lawful.
But always remember that no one knows what Law and Chaos really mean. They’re nebulously defined, and their relationship to landscape in particular is not often discussed in the books—Acheron, Carceri, and Gehenna are those alignments because those are the alignments the authors gave it, and if those authors have ever explained their choices, I’m not aware of it. Even if they have, their considerations are going to be highly personal and idiosyncratic—because everyone’s definitions of Chaos and Law are. There is a reason no one ever published a Book of Unfettered Discord or Book of Perfect Dogma—no one can pin down Chaos and Law well enough to write it. It’s all “I know it when I see it,” except everyone keeps disagreeing on when they’ve seen it. So ultimately, it’s unsurprising that these realms don’t seem to match their alignments to you, because your conception of those alignments is yours and more-or-less yours alone. D&D has a conceit wherein these alignments are objective, universal truths—but the real world doesn’t.
