There is nothing that isn't scary about crocodiles; on an evolutionary scale they're much closer to dinosaurs than to any other modern reptile, and I'm sure that the little chill I get when I see them is also an evolutionary response, handed down from my hominid ancestors.
But even more than those rows of sharp teeth and those blank cold eyes, it's their speed that's truly frightening. Though they lie basking in the sun on the riverbanks for hours at a time, barely moving, once they do slink into action, they're extremely fast. One second they're on the sand; blink and they're completely submerged and heading in your direction. I was always glad to be watching them from the safety of a very large boat.