Man, there are just so many drawbacks to a specie based currency, but a few of the top hits;

1. If you don't have control of the supply of the commodity on which you are "basing" your currency then you can't control its value. Commodity price is tightly linked to the supply/demand dynamic, and guess who controls much of the supply of gold, hint, it's neither us nor a friend.

2. You need to HAVE enough of the stuff "backing" your currency to coin enough to serve your economy. If you don't you have to inflate the value of your specie holdings to create the needed currency. The U.S. is the largest holder of gold by a factor of three, we have about $300 billion at today's prices, we have a $20 TRILLION yearly GDP.

3. Granting gold the value necessary to "back" enough currency to serve the global economy would start an epic gold rush...say goodby to much of the rainforest in both South America and Africa, not to mention the supplies of fresh water generated by those eco-systems, say hello to tens, if not hundreds of millions of mercury poisoned children pressed into ad-hoc mining operations.

There's just a few for a start.