I think international remittance will be the first killer use of Bitcoin.  International wires can eat as much as 5% in fees and it will take days.  I am sure major players like Coinbase and Circle can get this down to 0.5%-2% in the next couple of years.  Coinbase has already expanded to 13 European countries in just the last couple of weeks.  For anybody who is living half their time in Europe and half their time in the US, they can now get money from a Euro bank account to a US bank account (or vice versa) via Coinbase much easier, faster, and cheaper.  And who doesn't want to send money easier, faster and cheaper.  Somebody who gets paid in Euro, but needs USD can now get their Euros to USD via there iphone app in a very short time with small fees. When this catches on I think Bitcoin will see a big boom. 

The next step is expanding to Asia.  When Coinbase (or other similar company) is in Asia, then they will be able to spread the Bitcoin network across the three major concentrations of wealth in the world (Europe, North America, and Asia). 

International remittances is the easiest way for bitcoin to have a major presence in a certain area of the economy.  After that, then I think having shops accept bitcoin is the next step. 

My poor family member overseas needs money.  So instead of Western Union or a bank wire, I send bitcoin.  Normally they would have to turn this into cash to go shopping (losing 1%) but what if the store they are shopping in accepts bitcoin.  Then they just pay with bitcoin directly.  It is for this reason that I think that international remittance as a killer feature of bitcoin will drive shops into taking it even more than they do now and that eventually bitcoin will be come a real currency.