1) Global Economy -
Although BTC is heading towards major player in global economy on the other hand it is acquiring the place of national currency, Which is not good for the economies on micro level. No taxes, no government revenue, no funds for citizens (and yes corruption will not be monopoly of governments).
2) Thugs -
If Nigerian spammers can thug us even while using mainstream currencies they will be able to thug us in these virtual currencies too. In fact having global existence and operations of the currency is advantage for them.
3) Misuse -
Global phenomenon is also an advantage for the terrorists around the world. If bitcoin is making us more powerful so to them also.
I things governments will play huge role in the evolution and growth of these currencies.