According to Steve Forbes, bitcoin does not qualify as a currency. The basic reason? Bitcoins do not have a fixed value, thanks to their outrageous volatility:

Money is most optimal when it is fixed in value just as commerce is facilitated when we have fixed weights and measures. When you buy a pound of hamburger you expect to get 16 ounces of meat. An hour has 60 minutes. A mile has 5280 feet. These measurements don’t “float.”

Forbes makes a valid point. Cryptocurrencies in general don’t stand a chance of widespread adoption until the massive swings in daily (hourly) value calm way down. Nobody wants to worry about their own personal version of the $2 million bitcoin pizza every time they spend cryptocurrency, after all.