One major unfixable mistake is in using the gold standard for the D&D economy. It's bad history and it's bad game design. Historically, medieval money supplies were almost uniformly in silver coins, and the value was fundamentally based in how much raw silver metal was at hand. But more importantly for our purposes, in D&D PCs start out with the most valuable coinage, and will only experience currency going down from there. Clearly, it would be better to start with the cheapest metal, and then advance through more exotic categories. (In accordance with the central design conceit of D&D itself.) Even if some portion of us run games with a silver standard (for example, see Dragon #74, June 1983), the gold-piece premise has been reiterated in so many hundreds of RPG's and computer games that it's inseparable from fantasy in general in the public's mind.
Leaving that aside, let's consider the value of our ideal coins. The historical issue is enormously complex (see proverb above) because every principality used different mints, sizes, metal purities, units of weight, etc. But there are two principal issues that we're warned to keep in mind. One is the difference in units between actual coins versus moneys-of-account (i.e., units used in paper accounting only). For example, in the famous Carolingian money system (1 pound/louvre = 20 shillings/sou = 240 pence/deniers) only pennies/deniers were actually minted -- no shillings/sous or pound coins were created, and those units were used for record-keeping and ledgers only.
The second principal cautionary issue is that of debasement: Over the years, the princes and their mints would continually reduce the amount of silver content in their coins (by either mixing in more copper, etc., or reducing the overall coin size). For us, this makes it hard to compare the value of actual coins over the whole medieval period, because the values were constantly in a downward slide, in some cases causing inflation and the need to establish new, larger coins or non-debased currency. (In fact, this was one of the reasons to use "moneys-of-account" -- sometimes measuring raw silver bullion weight -- to keep a fair evaluation of one's worth, even when the coins were getting less valuable over the years.) One notable exception: England (see linked article), which kept the pound sterling mostly fixed over the years -- and therefore we might use that as the most dependable example of medieval coinage.
So, consider this particular historical example. View a list of English coinage in the 13th-15th centuries here. As our chief examples we'll take the "groat" (silver coin, worth 4 pence, i.e., 1/3 shilling) and "noble" (gold coin, worth 20 groats, i.e., 1/3 pound). There are smaller coins than these (half-nobles, quarter-nobles, etc.), but none significantly larger. So the noble:groat:penny coin ratio is 1:20:80.
I think this would be excellent from a game-design perspective, assuming that we used the silver standard as a basis. Two notable advantages: (1) The copper/silver pennies, at a 1:4 ratio, are not so cheap as to be entirely worthless and left in the dungeon by our adventurers. (2) The gold nobles, at a 1:20 ratio provide a nice geometric increase in the value that can be carried at higher levels, without having to resort to omnipresent bags of holding. Once again, the historical solution could serve as our game-design solution: copper coins for peasants, silver coins for the daily trade of freemen, and gold coins for transactions between kings.
Note also how close this historical coinage is to the OD&D system in Vol-2, p. 39, which stipulates a gold:silver:copper piece ratio of 1:10:50. It's basically a quasi-decimalization of the 1:20:80 ratio that we're finding in our research. In addition, it fairly represents the medieval valuation of gold bullion (about 1:10 to 1:14 of silver), assuming that all of our coins are the same size. The OD&D numbers are both reasonably good history and game design.
So, coming back to our game's history, why change those numbers in the AD&D Player's Handbook? In that work, Gygax establishes a 1:20:240 valuation for our game coins, while maintaining a gold standard. That turns all of our advantages into disadvantages: (1) Players start at the top value with nowhere to work up, (2) PCs are unable to carry much value in coinage at higher levels, (3) Copper pieces are effectively worthless. While using the classic Carolingian value ratios for the pound:shilling:pence, it overlooks the historical fact that those were not coins, but rather moneys-of-account only. A highly questionable change to the game, when the Original D&D system was so eminently reasonable in both historical and game-design terms.
So at this point, I'm personally a bit torn. If we were to establish a reduced-value system of coinage with a silver standard, which is preferable? The 1:20:80 ratio we see in historical England, or the 1:10:50 ratio we see in OD&D Vol-2? Poll completed; follow-up over here.