I have been watching Buffy lately, and I've realised that the only monsters there I am properly afraid of (as in, I'll skip the episode or hold hands in front of my eyes) are the guy in Killed by Death and the gentlemen in Hush. And the more I think about it, they follow a trend in how I react to monsters. If there is something wrong with the eyes or they smile wrong (my main problem with Hufsa in the Moomin), I am petrified. That is all it takes. It in addition they are tiny children or float instead of walking, I will sleep with the light on, but the core of the problem is eyes and smiles.

Is this just me? What makes a monster scary?
I may stop reading replies if they get too scary.