Cancelling transactions
One of the fundamental principals of cryptocurrency is to be able to operate without the need of a trusted central entity.
I can't see how a transaction can be cancelled or reversed without involving a trusted central entity to arbitrate the transaction.

However we also have cryptocurrency like Ripple. In my opinion Ripple is not a cryptocurrency because of the centralized nature, but well what do I know.

Maybe in the future some genius will discover a method to implement this without a central entity :)