You want to write something related to (say) a share price, or property ownership? Then your code needs to trust an external system providing the data, and the users need to trust that other people will honour the 'facts' decided by your program on the blockchain.
You want to create even a simple system that opens a lock when someone pays money? (The much-hyped Slock.it startup selling 'smart' locks) The blockchain makes no promise that a physical lock will operate based on bits in the blockchain. You have to trust that someone isn't cheating you.
And why is this need for extra trust a problem? Because once you require trust, you are back into the real world where we already have real solutions to these Ethereum based problems, that rely on trust (e.g. Share dealing, property sales, etc). By moving onto the blockchain, you have gained nothing and only complicated matters.
It's like the IoT: putting a blockchain into an existing system is just as helpful as wiring your fridge up to the Internet. For anyone getting seriously involved in Ethereum, sorry for your loss.