Presumably smart contracts would very often depend on input external to the system - for example, imagine a will that says if Person A dies, Person B inherits the millions of dollars worth of Ether at a particular address that Person A owns.

Somebody (or some other system) has to be in charge of the input to the Ethereum system to tell it that Person A has died. This seems like a huge single point of failure that doesn't exist with current contracts, if it gets hacked then the money gets transferred irreversibly and that's that.

Trying to build a smart contract with any kind of "real world" dependency like this seems equivalent to using a crypto system with an extra key that the government controls, there's a single point of failure that could compromise the whole system.