Unless you live in a US/Europe-centric bubble, the average Joe doesn't even have a credit/debit card. The average Joe however does have a cell phone, and many people even have smart phones. Other phone based payment systems in developing economies charge fees near 20%. Bitcoin will blow them out of the water, while also opening up new markets to existing online merchants. It will also reduce the barriers to entry for new online merchants that wish to serve these markets.
Many developing countries have relatively cheap domestic payment systems like Mpesa in East Africa or Alipay in China. I don't see bitcoin forming a threat to those systems. Fees are nowhere near 20% and you don't have to worry about the insane bitcoin volatility.
The transaction charges range is from 66% down to 0.16% depending who is involved and the amount of money:
up to 66% for a transfer to an unregistered user
up to 30% for a transfer to a registered user
the lowest transfer rate is 0.16% to a registered user for KSH 70'000 (800 USD, 580 EUR)
the cost for withdrawing money from an M-Pesa agent reach from 20% max to min 0.47%