Playing God?

A familiar kind of moral objection to euthanasia, physician-assisted suicide, abortion, cloning, stem cell research, and the like, is that they involve "playing God." What exactly does it mean to play God? Why is it playing God when we help someone die by giving them a lethal injection but not when we help someone live by giving them radiation or chemotherapy--or even an organ transplant? Is playing God always wrong, or only sometimes wrong? And why do we think playing God is wrong? After all, in many contexts, we think of it as a good thing to be like God--for we are made in the image of God.