Bitcoin's an interesting experiment but it's ultimately doomed because of the increasing "cost" (in terms of CPU cycles) of generating new bitcoins. http://goo.gl/I34VL
The real problem with crytocurrencies is scaling transactions. In a model where every participant has to be aware of every transaction, it is extremely difficult to imagine it (quickly) handling millions of transactions a day.
Bitcoin economy should grow pretty big before scaling really becomes a problem. I think it won't be a problem for at least 1-2 years, unless things really explode. That would mean new world order :D
Some of the scalability issues are addressed here: https://en.bitcoin.it/wiki/Scalability
Try explaining that to the man on the street.