The money/currency rant, being the last of the rants from the previous poll, after which we move on to the ones from the poll I put up a few days ago.
(I am also trying to catch up on comments, which shall be done, if slowly).
This is question format. Because I can.

1) What’s it made of?

Say that the first page of your fantasy novel has your hero, Pasheron, counting coins out to pay the merchant for the expensive bracelet he just bought his lover, his heart fast sinking because it’s become apparent that he left the purse with all the gold coins on his nightstand. The merchant is waiting impatiently, your readers are probably somewhere between amused and sympathetic, and you’ve let them know that gold has the same value, or something like it, in this world that it does in our own.
But then on, say, the tenth page, Pasheron rides home, through a city whose streets are literally paved with gold, because it’s so common as to be worthless.
Let’s back up here, shall we?
You don’t have to have gold be the metal of your currency. It doesn’t have to not be gold, either. It doesn’t have to be metal (though there are some materials that are less advisable than others; bark and leaves, for example, would probably shred too easily if required to pass through many hands). As metal, it could be copper, or bronze, or pyrite, which is what I happened to choose for the most expensive currency of the first fantasy world I created.
But it really should be something that can’t be just picked up off the street. Then there’d be endless arguments about how much a gold cobblestone was worth compared to a golden coin, whether the merchant could pass the cobblestone to anyone else, who would keep thieves from snatching up all the cobblestones, and so on. It’s asking for trouble.
As with so much else in fantasy worldbuilding, this requires checks and balances, and, above all else, the ability to realize, yourself, when one part of the worldbuilding invalidates another. Decide which you like more, gold pavement or gold coins, and then sacrifice the one you don’t like as much as the other.

2) Who backs the currency?

This twines with point 1, really, and is especially pertinent for heroes who travel all over the place. If the land is divided among competing currencies, they might earn a fortune in Lord Dreshon’s money for exposing a traitor, only to travel into Lord Felshon’s territory and discover it’s completely worthless because the local merchants only accept local coins.
Of course, this may be exactly the effect you want, if you’re going for a war-torn country, something like Germany or Italy before they united, where local “princes” had power close to home and ambitions to rule more than their homes, without the power to enforce those ambitions. But if you have a kingdom, an empire, an oligarchy, or another system of government that claims dominance over a good portion of the surrounding land, it is political suicide for those rulers to let someone else have control of the royal/imperial/merchants’ mint, or to let someone else create rival coins. I don’t care what kind of justifications the author comes up with. The Byzantine regulations that would have to be worked out could make for a cool story, sure, but the chance that anyone would have let them grow up in the first place, if they had the power to prevent it, is slim. And the sheer annoyance to travelers, tourists, foreign merchants, and anyone else who came in from outside with money to burn would probably inspire the true rulers to suppress the false coins soon enough.
Money is a dream, essentially, based on a system of trust. Coins have the value that people say they do because there is someone, whoever controls the mint, backing it up. If that trust fails, the currency system falls apart completely (unless you worked out another way of using currency, of course, which would be interesting, but which I’ve never seen). So don’t attack that trust.

3) Who handles it?

I’ve read several stories now that include the character finding the secret to such-and-such in a system of accountants’ ledgers. This is fine, if such a system of accountants is part of the world. But quite often, the detail feels extraneous, added on because the author couldn’t think of any other way for the protagonist to learn the solution to the mystery she’d set up, which indicates to me that she hasn’t thought about the way money works in her world.
Do this. It’s important. And it should include some thought about the people whose job it is to deal with the money.
I think most people don’t do research into it because it’s not inherently interesting to most authors like, say, hand-to-hand combat is. But, if it’s necessary to your plot, you also need it. So research, and think.
Are there banks? They must be fairly well-trusted, then, if a lot of people keep money in them. (They may not be well-trusted; this was a common fear in the US in both the nineteenth century, when there were bank panics, and in the twentieth, around the time of the Depression). How does the banking system work? How long does it take someone to actually get the loans they need? It’s probably longer than in our world, since these banks won’t have computers.
Are there accountants? Who employs them, and where do they live? An accountant living in a city with a lot of wealthy clients would not be impressed if a sweaty warrior swaggered in his door and demanded his services, because he wouldn’t have to be. An accountant in a smaller city, or a competitive one, might have to be.
Are there pawnshops and fences? Probably so, since otherwise the thieves that seem endemic to fantasy settings wouldn’t have anyone to sell their goods to. The usual stereotype is to portray these people as pushovers and cowards who start blubbering the moment a warrior threatens them, which is silly. If they were really that way, they would make no money, since anyone could intimidate them into offering large sums for property worth very little. Consider who they are, how they work, and what sort of people they are when the heroes come knocking on their door.
Yes, I know it can seem really boring; tracing the progress of finance is not most fantasy authors’ idea of a good time. But the very intricacy of the operation demands that close attention. If you hang your whole story off something like this, and you contradict your worldbuilding, or—horror of horrors—your math is wrong, someone will notice. And, readers being who they are, someone will probably take great glee in pointing it out, too.

4) Where are the counterfeiters?

Oh, come on, you know there’ve got to be some. If a clever person comes up with money on day 1, a counterfeiter will come up with a way to forge it on day 2, and the clever person will find a way to try and prevent that on day 3, and the counterfeiter will find a way to get around the prevention on day 4.
What kind of protections the money will need and what kind of methods the counterfeiters will use to step around the protections depend on several things:

  • The makeup of your currency. Carefully milled golden coins will be harder to forge than misshapen gold lumps. (And of course you go back to the argument about how much a gold lump is worth again).
  • The magic/technology available. A fantasy world with technology on the Renaissance level is simply not going to be able to protect its money in the same way as the twenty-first-century US government does.
  • The rate of success in catching counterfeiters. Say all the king’s guards really are as stupid and bumbling as the author portrays them as. They may not catch any but the most stupid and obvious counterfeiters, leading others to become more bold.
  • What the punishment is for counterfeiting. If it’s a slap on the wrist, it’ll be more common. If it’s an execution, it might be less common. If mages spread a blanket spell over the kingdom to insure that anyone even thinking about counterfeiting the king’s coin loses their fingers, I imagine that people would be quiet and honest and terrified, or extremely sneaky and intelligent.

5) What’s valuable besides money?

In most fantasy settings, it’s gems. Dragon hoards consist of rubies, sapphires, emeralds and so on, probably still in their settings of cups and swords and so on, as well as golden and silver coins. The triumphant hero carries the ruby home, sells it, and lives a life of luxury.
You don’t have to limit yourself to that, though. Change conditions just a bit, and gems lose their value. If they’re very common, it might take a ruby of extraordinary size and fire to win a good price from an appraiser. If you have a fantasy species that has no mania for decoration and no technology that could use gems, what do they want with rocks? If several dragon’s hoards of rubies are discovered and unleashed on the market at once, the price of rubies will drop, because now they’re more (artificially) common. And, of course, if you can magically conjure up a ruby from thin air, there’s no need to go hunting all over creation for one, or buy it.
Ask whether gems must be the secondary tool of your currency system, which, in the absence of official government backing, becomes more like a barter system. A farmer may need a horse more than he needs a diamond, and will certainly need food more than a ruby.
This ties into point 6, so we’ll go there now.

6) What amount of money is common for people of different social classes to have?

If only nobles regularly handle golden coins, it would look really suspicious for a farmer to have one, wouldn’t it? And if the heroine is a peasant and is being evicted because she has no money, why in the world does she have a magical pendant that the author insists is worth more than her house? Why hasn’t she sold or pawned the pendant and saved her house? That the pendant is a keepsake from her dead mother, yadda yadda yadda, is far too overused as an excuse. Sentimentality comes to mean a lot less when you’re cold and wet and starving. That the pendant is magically welded to her neck is a better one.
Consider this for your thief character, too. If she’s lower-class and looks it, why would she steal expensive fur muffs from her victims? Now, she might have a good fence, who will give her a nice portion of what the muff is worth and move it on quickly. But her trying to sell it will look suspicious. People will notice and remember a girl in ragged clothes who wears a fur muff. If she has no practical use for it, and no fence, and it would mark her out, it’s stupid to steal it. And stupid thieves quickly get caught, or at least they should.
This is one place where many fantasy plots run headlong into common sense, because they want the heroine to be in trouble while keeping her magical pendant at all costs, or they want a thief who steals an item she has no use for but which propels her into an adventure. With a bit of stretching and straining, you might be able to make this work, but the more stretching and straining you have to do, the worse an idea it is. Come up with something that will seem effortlessly plausible.
It looks like avoiding the Designated Love Interest trap is next.