Azor's answer is thorough. I'll just add a few things:
There may be differences regionally. Saying "the Deep South" is a relatively obvious proper noun to an American and thus capitalized, but perhaps not other native English speakers would know this. There may also be differences within categories such as a pope pope versus the Pope. Pope. And yet some would capitalize Pope under any circumstances. Because of these variations, capitalization rules can be considered a function of style. A good style guide such asThe Chicago Manual of Style can be used to give the writer a consistent set of rules.
There are languages without case, such as Hebrew, that make no effort to distinguish the difference. To make it even more ambiguous, Hebrew lacks the articles a and an.
Having said that, my answer is: there is no consensus. The distinction between proper and common nouns is dependent upon the language, regional and cultural factors, and stylistic concerns. A style guide consistent within the language and region you are publishing in can direct you in a consistent way.