Learn English – Yoga (proper-case) or yoga (lowercase)

"Hell" is capitalized when it is used as a proper noun. That is, you capitalize it when you are referring to it as a specific place. However, it can be perfectly legitimate to leave it uncapitalized if you are not referring to a specific place.

"That job was hell" does not refer to a specific location, but rather a nebulous concept of torture. Thus, it is not capitalized in this sentence.

Proper nouns should name specific people, places or things - "specific" being the key word here. In some cases specificity is immediately apparent, e.g. in the term "Singapore Airlines" (after all, there is only one airline with that name!).

In other cases, there is a grey area and I would say that fame/notoriety also plays a role in determining specificity. To use your example, one would refer to the Guantanamo letters if they don't contain any content that is particularly newsworthy. If, on the other hand, these letters happen to disclose some salient information that could result in a political scandal, then these letters would quickly garner attention and become intimately linked to this political scandal in the minds of people. Such a link would imbue the letters with a specific quality that would warrant the use of a proper noun, i.e. the Guantanamo Letters.

Thus, some common nouns can become proper nouns over time as they gain specificity in the minds of people. This can be a very organic process that can leave plenty of room for interpretation as to whether something should be considered a proper noun or not.