Nightmare Retardant: Giant albino penguins. They are described as six feet tall and eyeless, shambling about and squawking loudly, a far cry from this◊. A bit unsettling depending on the reader.
It's treated as such by the narrator, as while they are exploring the caves and at first see them only as vague tall white shapes, and for a moment are quite frightened. They are then revealed to be the penguins, a reveal which the narrator describes as "a flash of anti-climax." He notes they're still pretty unnerving, describing them as "monstrous".
Slow-Paced Beginning: Fifty pages of description of how their scientific expedition was meant to go. How it actually went starts around page sixty.
Values Dissonance: The reader is meant to sympathize with the "upper-class" Elder Things over the "lower-class" Shoggoths, which were manufactured en masse by the Elder Things to act as their servitors. Admittedly, it's easy to do when one is rational and ill-fated and the other is a formless, possibly non-sentient mass of black goo trying to rip your head off (literally), but still. It's like a biological ancestor to the Robot War.