Someone rather more partial to Ron Paul's arguments in favor of the gold standard than I am asks me to write a post outlining my objections to it. All right, here goes.
Money is a mysterious thing. It is a store of value, it is a medium of exchange. It is, in a fiat currency economy, worth only what people think it is worth, and what they think it is worth can be oddly affected by what they think it may be worth in the future, resulting in self-fulfilling feedback loops (at least in the short term). Even in non-fiat currencies, such as the gold standard, the value of the underlying asset can be changed by rising (or shrinking) demand for money. Economists studying this fascinating topic tend to suffer from migraines as they suffer from all the mysterious--hell, nearly mystical--attributes of money.
However, over the last fifty years, economists have settled on some very broad areas of consensus. The first is, as famous libertarian monetary economist Milton Friedman wrote, "inflation is always and everywhere a monetary phenomenon". When the supply of money outstrips the demand, prices rise. And this is by no means limited to fiat currencies; see the great Spanish inflation of the 16th & 17th centuries, thanks to the steady influx of gold from the New World. Or check out the price of basic commodities in mining towns during the Gold Rush, when all anyone had was gold.
The second is that a little bit of inflation is okay--possibly even beneficial, since it helps the economy to overcome the problem of sticky wages when the relative value of labour has fallen. But a lot of inflation is very, very bad. Exhibit A is Zimbabwe; Exhibits B-∞ are every other economy that has had inflation near or above the double-digit mark; the higher the inflation, the worse the economy did. The feeling that the currency will experience an unpredictable amount of inflation dampens the willingness of the citizens to save and invest, which is why so many third-world loans are denominated in dollars.
The third is that deflation is also bad, and at the lower percentage values, often even worse than inflation. This surprises/offends/meets with the frank disbelief of many "sound money" types, who think that, barring local shortage, in an ideal world everything ought to cost the same or less than it did when Grandpa was a boy. (These sorts of opinions are cemented further by the fact that Grandpa, who is often the source of them, is usually living on a fixed income, and therefore feels that he would make out better in a deflationary economy.) The problem is, deflation does rather devastating things to anyone who has debt, since they now have to repay what they borrowed in more expensive dollars. Deflation means that, thanks to the abovementioned sticky wages, the economy has to deal with demand shocks by lowering output. Deflation can result in what's known as a liquidity trap, a concept pioneered by liberal economist John Maynard Keynes and best elucidated by liberal economist Paul Krugman back before he left economics writing to focus on his hatred of George W. Bush. Deflation is what made the Great Depression so memorable. Deflation is so bad that almost everyone agrees that moderate inflation, in the range of 1-2%, is better than risking even a small amount of deflation.
Advocates of a gold standard dispute this. They argue that America experienced a long, slow deflation throughout most of the 19th century, without anyone getting hurt. What they neglect to mention is that people did get hurt, repeatedly, in the period's awful financial contractions. Though we don't have modern economic statistics for the period, it's pretty clear that recessions were longer and deeper than they are now.