If a reader were to open an English book printed in the 17th century, one of the first things he'd notice would be the unconventional spellings, and how most nouns were capitalized. Just like the German today, it was customary during the late 17th century and the end of the 18th century to emphasize all the nouns (or nearly) with a capital letter, but there were no fixed rules governing this practice. It appears to have been a fashionable trend that became increasingly more idiosyncratic and terribly subjective.
In German (and Luxembourgish), all nouns are capitalized. This was
also practiced in Danish before the spelling reform of 1948, and in
English during the 18th century (as in Gulliver's Travels, and most of
the original 1787 United States Constitution).
First names, or more formally, personal names and proper names/nouns were indistinct from ordinary nouns; it was primarily the writer's choice which nouns to capitalize or italicized.
Eighteenth century grammars
In the early decades of the eighteenth century, we get some sense that
printers found this abundant capitalization unnecessary. Thomas Dyche,
in A guide to the English tongue, writes, "'Tis grown Customary in
Printing to begin every Substantive with a Capital, but in my Opinion
'tis unnecessary, and hinders that remarkable Dinstinction intended by
the Capitals". And Thomas Tuite agrees with his claim
that "[substantive capitalization] hinders that expressive beauty...
intended by a capital" . With opinions changing and
printers altering type font on their on accord, it was only a matter
of time before lower-case nouns appeared more frequently in literature
and other forms of writing.
This change didn't happen overnight, but generally speaking common nouns went from being capitalized to being written in lower-case; important nouns went from italicized-capitalized to italicized, and then lower-case; and certain proper nouns went from small caps to capitalized.
An example below of how some common nouns were capitalized, e.g., Divorce; Queen; Preachers; some were italicized-capitalized e.g., England; Katherine (of) Castile; Dutch-Land. And some were left in lower-case e.g., wife; fingers; hatred. The excerpt is taken from The memoires of Sir James Melvil of Hal-hill by George Scot (1683)
Source: Ask Meta Filter: What is the History of English Capitalization?