Wow, there have been some really great answers to this question.
Me, i'm not sure if i'm afraid of death or not. At the moment i'm wrestling with quitting my normal successful job in academia and joing the marines. I guess if i do then i'm fairly unafraid.
Actually, I sort of know the answer already. Death? I don't mind the prospect. I am terrified of serious injury though, like losing a limb. I love sport too much.