Despite the movie being over sixty years old, the ants are still rather frightening to look at.
The fact that the ants aren't introduced until about halfway through the movie contributes to this, as the sizable build-up to the ants' appearance makes their introduction all the more effective.
The trailer as well. "...with things so horrible...so terrifying...SO HIDEOUS! There is no word to describe THEM."
There's also the movie's blood-red title, which actually made it into the final movie.
Before we know what's really happening, there's the disappearance of the little girl's family. We don't know what did it, yet, but whatever it was, it drove her into catatonic shock. Plus the horrible death of old Gramps. And his rifle being twisted into a pretzel by the unseen killer. If you've seen it you know who and what is responsible, but if you haven't?
