The difference is that you can actually buy things with USD.
Everyone else's saying this: the value of Bitcoin is either zero or infinity[]. This was clear way from the beginning, and while I regret not cashing in on the bubble before.
[] This assuming the normal continuation of capitalism and the global concert of nations where there is openness to commerce and multiple fiat currencies. Of course, in Mencius Moldbug's dream (who first called for something like bitcoin in '08) where strong authoritarian government prevents other currencies from hanging on... it's an utopia. We might as well trade in time credits.
This is a pretty bad example but if I write a check to you for $10 I have given you a $10 note. It's backed by my promise to you that I have the money. You cannot buy anything directly with the check. It's made out to your name. All you can do with that currency is trade it into a bank for another currency we all use.
Another example may be gold. If I have you a gold bullion I hardly think you'd have an easy time spending it at a store (unless you're in Dubai). This doesn't mean gold doesn't have inherent value and it also doesn't mean gold is not a currency.