Great whitepaper explaining the visceral reaction we have to certain automata and movie monsters. The thesis is that things that look a little human aren't creepy, and things that look exactly human aren't creepy, but things that look close to human are really, really eerie — think of shambling zombies versus gorillas.
The uncanny valley itself is where dwell monsters, in the classic sense of the word. Frankenstein's creation, the undead, the ingeniously twisted demons of anime and their inspirations from legend and myth, and indeed all the walking terrors and horrors of man's imagining belong here. In essence, they tend to be warped funhouse-mirror images of humanity, and many if not most share one or both of a pair of common traits.
The more obvious of these is overt, intimidating superhuman power, whether physical or paranormal, but the other is far subtler. Recent research suggests that the human idea of beauty may rest on a surprisingly simple foundation: symmetry. According to the study, symmetry of face and body suggests health and vigor — and therefore genetic fitness — while asymmetry implies the opposite.