Open Source Romerun
Open Source.
I think the core concept of 100% transparency prevents cryptocurrencies from scaling to VISA-like volumes. Everyone can't hold a full record of everyone else's transactions worldwide, the block size would get too large to download a chain in real-time, let alone catch up.
But I don't really see that as a huge problem, since VISA doesn't print money. IMHO Bitcoin should be replacing the federal reserve banks, not VISA. Long ago, gold was traded much like bitcoin is today, but eventually the transaction costs made bank notes more practical. I think history will repeat itself, and we'll trust Ripple or Open Transactions networks for everyday business. Those organizations will be the ones paying high bitcoin transaction fees, but to move massive amounts of money belonging to multiple customers at once. With OT, this has the added benefit of truly anonymous digital cash without needing a mixing service.
What will be different this time is that we'll have a publicly-auditable monetary base that you can download on your phone, to be SURE that your old bank sent REAL uninflatable money to your new one. It won't be physically vulnerable like gold is, and won't be subject to seizure and eventual domination like gold was.