Should “Hell” be capitalized?

"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.

Hell need not be capitalized, even when it refers to a specific place, in the same way we need not capitalize equator (see for example Larry Trask's Guide to Punctuation).

The Cambridge Advanced Learner's Dictionary also allows for it to remain uncapitalized. Its capitalization then is possibly due to its religious significance and not its reference to a specific place.

Checking several bibles (that is as real as hell can be) also shows that it need not be capitalized.