There are some limitation which stop bitcoins from being used as a currency. For instance, the transactions are not instant and it is a big limitation in my opinion.
I don't deny that there are some limitations that bitcoin is facing at the moment but from my point of view, I don't suppose that transactions speed is one of them. Maybe it's not that immediate but it's still faster and easier to make transaction with bitcoin than the traditional ways. I think that what prevents bitcoin from being entirely a currency is that many countries are still having misleading thoughts about bitcoin and some even ban it. I hope that one day the governments would change their minds and bitcoin would eventually become an international currency.