Scott in the later part of the film. He pretty much admits that he's becoming a jerk the smaller he gets. However, considering what he's going through, it's hard to be but so angry at him.
An understatement in this case. In the novel Scott's sexual frustration is addressed in a more straightforward fashion.
(Novel only) When he is shorter than his daughter, Louise hires a 15 year old babysitter to watch her while she works. Scott while not making any direct moves, lusts over her (unknown to her as she doesn't know about his presence). Incidentallly, he vividly repeats a phrase he heard from a pedophile who mistook him for a little boy.
(Novel and comic book adaptation) Though he's upfront with Louise about his intentions, his relationship with Clarice is a one stand as he will eventually shrink to the point where intimacy will be impossible altogether (Unlike the film, there were no successful attempt to temporarily prevent the shrinking).
Scott's battle with the spider. Even though the film was made on a shoestring budget, the scenes with the spider are very, very creepy and undoubtedly were intended to be scary. It is indeed very scary even over fifty years later.
The entire beginning of the situation, especially in the original story, is extremely creepy. Imagine losing a 7th of an inch every day. Nothing major at first, but as time goes on you become smaller and smaller, with no way to stop it.
Visual Effects of Awesome: Even though it was on a shoestring budget, the effects (especially for a film from 1957 and in black and white) are fairly impressive. There's an occasional "scale" problem, but most of the time, it's done very well. In fact, the fight with the spider is done a little too well!