Well, at the moment with minecraft currently, I'd have to say 8/10. When you're just minding your own business mining some diamond, near lava, and suddenly you hear a 'ssss'. You just know you're done for. The creeper blows up, lava floods the hole, and you die a horrible demise, loot lost. When the endermen come in the next update, I expect it will bump it up to a 9 or a 9.5/10. The thought of them just staring back at you until you look away is definitely creepy, because you know they'll attack, and you know that they will get to you quickly. I guess its the fear of knowing what can and will happen that is what makes minecraft so scary to play.
