Just from a basic economic viewpoint, ignoring the scamming issues (because paper currencies have the same problems):
- If the amount of currency is fixed, then it means it can't expand along with the economy, causing deflation: since there is more value in the economy but the same amount of coins, that means that every coin will be worth more and prices will drop, causing in turn debts to increase in value as "X coins" will be harder to adquire as you get paid less and less for your work. At that point, it's almost better to just tuck your money under your bed, as you'll be able to buy more with it in the future... which kills the credit market (because why risk not being repaid when you can win just by saving?) and causes the economy to die down. In other words, economies cannot grow in a substantial way unless cash is poured into the system along with it.
-If the amount of currency is tied to *value of x*, then it's the gold standard all over again- maybe it will lose value when you need the opposite, or stand still when you need to increase it. It's essentially giving the keys for something as vital as your currency to a random number generator.
-If the value of the currency is chosen by the market, it becomes stock trading, which is one of the main problems bitcoin is suffering from right now: its value fluctuates nonsensically and wildly, which makes it impossible to any sane person to keep track of the prices every day. How annoying would it be if you had to check the exchange market every morning just to figure out how much to ask for a kilo of oranges? Not to mention that it could drop any day so you'd rather spend all you've got that very day and not risk losing everything of a sudden.
-If the currency is only valid in your country, you are killing foreign trade because most people will ask you for some sort of international tradeable currency when selling you oil, food or whatever most countries need to buy on a daily basis. You'd end up being forced to resort to barter. We have tradeable currencies now because non-convertible ones were a huge failure as soon as countries left protectionism around the 19th and 20th centuries. It would be essentially going back 100/200 years.
-If the value (and amount) of the currency is chosen by a group of people, well, I'd rather choose a democratic government to handle it- they might be stupid but at least you can remove them if they fail too bad. The other option is choosing a self-appointed creator who might or might not have any clue of how economy works or care about what's better for him instead of the country- and unlike a selfish Government, this self-appointed creator has no "term limits", remember. Having a Government in charge is "the worst option except all the others" all over again.