> "Semantics" have nothing to do with it. Commodities [0] != currencies [1]
Semantics have everything to do with it, as I will demonstrate below.
> One exists as a medium of exchange, while the other is a good with no differentiation between sources.
Well, as long as the USD is not fake, you don't care what the source is :) Similarly, for non-fake gold.
> Currencies are differentiated by the trustworthiness of the governments that issue them.
Today, that is true. However, the Chumash people had a currency that -- like bitcoin -- was not centrally managed, and reflected work put into something, rather than any fiat or backing store of value. http://en.wikipedia.org/wiki/Chumash_people#Culture - I can't remember where I read a more detailed account. Basically, you could sit on the beach all day and make money -- and it took slightly longer than the equivalent exchange rate would get you in food (so you could hunt/gather food, or make money, with comparable time expenditure, and exchange them). The whole system broke down when European drills (as manufacturing tools) were introduced and made money making simple.
> First: I admit I wasn't aware that retailers accepted bitcoins when I posted that comment. I've discovered otherwise now, so the "hill of beans" comment is wrong. Sorry.
So - let's get back to semantics: do you agree bitcoin is currency? (exists as a medium of exchange), or commodity? (a good with no differentiation between sources) because it has both properties, thus showing that the definitions are not mutually exclusive. A USD is currency but not commodity; Beans are commodity but not currency. Gold, Chumash beads and bitcoin are both.
> Name a first-world country where gold is widely accepted as currency.
In NYC where I live, hundreds of stores will accept your gold in exchange for other merchandise (at a bad exchange rate for you...). You can identify them by the sign "We buy gold". You must have seen them in other places too.
> but it's not backed by a central body
True, but ....
> nor does it carry a relatively stable value (unlike fiat money)
Wrong, and if you truly believe that you might be beyond help. The US devalued the USD by 50% in 1931. Belarus did it to their fiat currency last week. Between 2002 and 2006, the EUR/USD exchange rate went from 0.8 to 1.5 (almost 100%) - "stable"?
In fact, up until 1971, the backing for the USD (and most other currencies) was gold - it only became a true fiat currency in 1971! Before that, it was a claim on some amount of gold in fort knox, and value was derived from that.