Vancian Monsters
Much has been made of Vancian magic & its impact on D&D, but after reading The Dying Earth series the monsters have left an even more lasting impression on me. (I alluded to this in a previous post). I think there are two main reasons:
First, if I were going to write a story in which a character gets attacked by a bear, I wouldn't bother to spend a paragraph explaining what a bear is, what it looks like, why it's here, etc. But I might mention its enormous paws, it's shaggy coat, its height, or its gore-encrusted claws -- things that would make your pre-existing mental image of a bear pop for the sake of the story. This is basically how Vance writes his monsters, but instead of a bear it's a pelgrane or a deodand and we as readers have to assemble our own mental image. This paradoxically creates the sense that his world is alien (to us) and natural (to his characters).
Second, monsters in The Dying Earth are ambiguously human, usually capable of speech, sometimes wearing clothing (iirc the deodands wear some kind of leather harness). They engage in the same bizarre punctilious arguments as the human characters. Science fiction often unconsciously projects a human-animal dichotomy onto aliens, but in The Dying Earth Vance hews closer to a folkloric mode where the boundaries are blurred. Read against the prominence of vivimancy, there's a disturbing possibility that these sentient monsters were once human.