Just in case anyone is wondering about crypto currencies (bitcoin etc.), and how they differ gold/commodities or more traditional payment systems (and why I'm negative on them).
The actual promise of crypto currencies are as an alternative to a traditional payment system.
Traditionally, the transaction fees for electronic payment mechanisms are paid for by either the person spending the money (rare, b/c the person spending the money will pick another payment option if one is available) or by the person accepting the payment (who can then pass it on to the spender).
For crypto currencies, there is a store of wealth (the cash value of the 'coins'), which acts as a buffer and hides the transaction fee from both the person spending and the person buying. E.g. to buy something with a crypto currency, you first spend local currency to buy 'coins'. Then, spend the 'coins' in exchange for goods or services. Finally, the goods/service provider converts them back into their local currency. Holding the crypto coins for any length of time exposes the person accepting them to crypto-currency risk.
So, who pays the transaction fee? The people holding crypto currency 'coins' for the long term. When I looked, the effective rate was about 3.5% (which is high for a transaction fee).
To understand how that works, let's look at how crypto currency 'coins' differ from gold or more traditional non-monetary stores of wealth. The main difference is that between crypto coins and gold or other commodities is that there's no physical object corresponding to the crypto coin. So, instead of transferring the value via a physical exchange, the value of the crypto coins are transferred via a cryptographic algorithm and a validation service provided by crypto currency 'miners.'
The 'miners' typically run significant server farms and are rewarded for providing the service by being granted a fraction of the existing coins for whomever finishes the validation first.
Which brings us to a basic economic problem. For crypto currencies to be effective as a payment mechanism, there needs to be a liquid way to convert between local currencies and crypto coins. Like the Nasdaq, the liquidity is provided by people placing limit orders (I want to buy X coins at Y price and I want to sell X coins at Y price), and executed by market orders.
This creates two problems. First, there is no proper 'exchange' for crypto currencies, only private companies providing proprietary solutions, and many of those have proved to be insufficient (mtgox anyone)? Additionally, the people placing the buy side limit orders need to be interested in holding the crypto currencies for the long term (which means they want to pay the transaction fees on purchases they are not involved in). While an exchange may eventually exist, it is unlikely that there will remain a significant interest in paying transaction fees for other people.