Annotated in the thread of your forum, but the best descriptions of Acheron and Carceri that I have seen, based on the actual text, but not directly cited, are those of Jade Ripley (who speaks of Lord_Gareth here and many other places, including GiantITP) . com forums):
Acheron does not have architects. None of the exterior planes do it. […]
Yes, Acheron looks a bit chaotic on the outside. You know what, from the outside Carceri looks a bit licit. But Acheron is not 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; in fact, it is Acheron's total indifference to you that eventually kills you. Acheron is the annoying monotony of despair, and it is the tired horror of cynicism so great that it consumes morality. The sergeant who tires of fighting corruption and accepts bribery goes to Acheron; The office drone that eliminates their misery towards others by providing barriers to real help goes to Acheron. It is the punishment for which there was no crime, the penalty without violation, the monolithic indifference of the Law without moral compass, of conflict without belief, of tyranny without vanity.
Acheron does not hate you.
He wants you to die anyway.
Carceri, the red prison, is a plane of injustice. Carceri is a mockery of the law and of legal thought, seen through a lens of powerful and counterproductive malevolence. Carceri is full of vain struggles, miserable anguish, trapped rage and a heady mix of hope and despair that drives its inhabitants crazy. From the outside, Carceri seems lawful, it is a prison, after all, but what separates Carceri from ordinary prisons is that no one is in charge, and despite deception to the contrary, no one can be in charge. Although the inhabitants of Carceri make an effort to separate the "prisoners" from the people who "simply live there" (that is, they are not locked in a specific prison complex, prison structure or torment), the truth is that everything is in Carceri. They are prisoners, trapped there by their own fear, hatred, distrust, paranoia, vices, greed, desires and malice.
And, really, that is Carceri's drive. The Red Prison mocks you with hope, offers to escape and only gives frustration and despair. Even if you leave, it drags you back, proving that any escape is merely temporary. Their "guards" can not leave, and they can not impose order on their prisoners beyond the reach of their weapons. Riots and murders abound, a great mass of hatred and frustration that shakes the bars and shakes the cages, echoing through the plane and mingling with cries of pain and pleasure. Actually, it's not the plane that keeps you there, it's you.
It was always you. It is the biggest and cruel irony of Carceri. In the end, the reason he will never leave is that even if he was ever fit for society, he never will be again. You can leave Carceri, but Carceri 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 everything else. It is an arid landscape, and that uniformity is quite legal.
But always remember that Nobody knows what Law and Chaos really means.. They are nebulously defined and their relationship to the landscape in particular is not often discussed in the books: Acheron, Carceri and Gehenna are those alignments because they are the alignments that the authors provided, and if those authors ever explained their options, No I am aware of it. Even if they do, their considerations will be highly personal and idiosyncratic, because the definitions of Chaos and Laws of all are. There is a reason why nobody has published a Book of discord without restrictions or Book of the perfect dogma– Nobody can specify Chaos and Law well enough to write it. It's all "I know when I see it", except that everyone still disagrees when they've seen it. So, ultimately, it's not surprising that these realms do not seem to match your alignments with you, because your conception of those alignments is yours and, more or less, yours alone. D & D has the presumption that these alignments are objective and universal truths, but the real world does not.