1. There is very little you can buy with BTC at this time.

2. While BTC transactions are free, anytime you try to convert your BTC into real-world currencies, people take a huge commission.

3. In the traditional credit card payment system, the fees are assumed by the vendor.  Using BTC, both parties are charge fees to transfer real-world currency into BTC, and then BTC into real-world currency.

4. It is technologically complicated to buy and sell BTC.  Most exchange sites only deal in money orders, wire transfers and other methods which are costly, take a lot of time, and are insecure.

In conclusion, before BTC can become a mass-market currency, it will have to be much simpler to buy / sell BTC.  You will need local branches of exchange sites such as Mt Gox all over the world.  You will need a card system which people are familiar with that looks and feels like a credit / debit card.  And finally you will need a larger amount of vendors that accept BTC, especially the large multi-nationals where people spend billions of dollars in.