It needs amazon to adopt it. That would probably send the price up ten fold and it would make sense for them to compete with eBay/paypal but I guess as the moment it's not stable enough.
Biggest problem it has IMHO is the anonymous nature of transactions makin it hard to prove you have paid but if amazon acted as a middle man in their sales then they would create that traceability.
Couple years off at least though I suspect.
There are payment processors like PayPal in the works, so it will definitely be interesting.
Proving you have paid is easy - generating addresses is free and easy - you generate an address per transaction, if it gains the desired coins, they've paid
It works the same on most places, from exchanges to the existing sites that accept BTC so far.