The fact that it doesn't require a pin or any kind of authorisation (once you've put the details in once) unless the purchase is over a certain amount, ?50 I think.
Theoretically I could take your NFC phone or NFC credit card and go and make repeated transactions of ?49.99 without having to know your details.
I like the idea in theory as I'm obsessively careful with my phone/wallet, but still wouldn't risk it just in case. It'll be a disaster/nightmare for many people less aware; as if people needed more of an incentive to steal someone's expensive phone, now they can use it to buy stuff too! Woo! Technology :dry