These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Also in Perelandra the underground section can be incredibly creepy. Not just because most of it takes place in total darkness, but because there are lots of hints that all sorts of bizarre persons and places exist down there and the reader only sees enough of them to hint at it. And what little we see is hinted to be not necessarily evil but so foreign to human experience that it's incomprehensible.
The detailed description of the head and the creepy paintings in That Hideous Strength. Also Dr. Frost's POV segments.
Older Than They Think: That Hideous Strength addresses the issue of transhumanism and many of its implications. Its first printing was in 1945.
Seinfeld Is Unfunny: At the time the trilogy was first published, most aliens in SF stories were hostile savages intent on destroying humanity. For Lewis's aliens to be morally superior to man was a radical departure... which was widely adopted by later writers, somewhat diluting its impact today. Though, the thing that remains the most original with Lewis' premise is that nearly any other time aliens are morally superior, they tend to not believe in any Deity.
Values Dissonance: You're in a theological debate. You find yourself losing the argument and fear that your audience might be swayed towards the opinions of your opponent. What do you do? If you're Ransom, you give up arguing the point and just kill the dirty satanic villain with your bare hands.
Ransom himself thought that particular solution seemed crass, until he realized that his opponent was playing similarly dirty. The Un-man was using manipulative rhetoric to make his point, and was abusing the fact that he didn't need to sleep in order to physically wear down his opposition.
You must also take into account that A) Ransom's opponent was literally Satan. There was no possible meeting of the minds or compromise here. It was victory and life or defeat and death (of an entire world). And B) This was not his first solution, but his last. He came to this conclusion only after being dragged through weeks of argument. It also required tremendous courage. Ransom fully expected to die, as he was a mortal entering physical combat with a demon. He quickly discovered, to his relief, that it was, physically, one middle-aged scholar against another. And Ransom was a trained boxer.