i would be least scared of a blade flying off. as Helijack said lots of things can go wrong and do go wrong.. having the thing twist out of control and slam me or much much worse someone else or their property is what worries me... people have been killed on more than one occasion by an out of control rc helicopter.
Some vids circulating the net don't help the promotion of safety matters either. you have guys flying them under and into shade covers , other's clowning around by tieing them to fishing poles etc... ridiculous.