Shouldn’t Heaven and Hell be capitalized, as they are the names of places?
No less an authority than Fulton Sheen had the galleys for his latest book come back from the typesetters with “Heaven” and “Hell” knocked down to lowercase. He carefully re-capitalized each occurrence. When his editor called to request an explanation, he gave what I think we can regard as the definitive answer to Nicholas’s question:
Because they’re places. You know, like Scarsdale.
I don't think either one is a proper noun unless you're talking about a particular version. Here are illustrations:
In any afterlife, one would expect there to be some form of heaven.
The writer is clearly talking about a general concept.
500 years' walk beyond the edge of the universe lies the sphere of the angels. 500 years' walk beyond that lies an altar where the archangel Michael offers up the souls of the righteous. 500 years walk beyond that lies Heaven.
The writer is clearly talking about a specific place. He's even giving directions.
Jesus said that people who do xyz will go to hell.
Ambiguous because while it's clear from context that the writer is talking about a particular Hell, the sentence structure iself would work just as well if hell were a general abstract concept.
If you refer to a unique place, such as Earth, then you would capitalise it as befits a proper noun.
If you refer to a class of places, such as the city or the country, then you would not capitalise the common noun.
Now you might hold a view that there is one true heaven, and as such deem it unique and worthy of capitalisation. Or you might admit the possibility of many heavens and place it in the category of common nouns, not worthy of capitalisation.
ERRATUM: The comments to my answer helped to change my mind on this. I now agree that heaven and hell should always be lowercase.
I believe they should be capitalized when used as place names. I've noticed a tapering-off in their capitalization in my lifetime. I don't believe in either place myself, but belief doesn't enter into it, as we capitalize plenty of fictional and mythological place names -- e.g., Narnia, Vulcan, Hades.
I suppose there is an argument that, because Heaven and Hell can't be categorized as cities, planets, countries, etc., they don't deserve to be proper nouns, but that seems to me a weak argument (especially in light of Hades, Valhalla, etc.). They are places, albeit mythological ones, and there is only one of each.
In some instances, heaven and hell are used in figures of speech, such as: "Thank heaven." "What the hell?" In those instances, lowercase works fine.