I’ve just handed in an essay about the influence of D&D on fantasy fiction.
The ‘standard fantasy setting’ seems to be quite influenced by D&D. At least, it has a lot of features that are found in D&D, and never or rarely found in Tolkien or other pre-1970s writers.
The main ones I found were:
Fantasy worlds with no connection to the real world That is, they’re not stated to be in an imagined past (like Lord of the Rings) or future, on another planet, in an alternate history etc.
Many species living more or less peacefully in a single civilisation As opposed to Middle Earth, where everyone lives apart, often don’t even know of each other’s existence, and if they do interact tend to hate and kill each other.
Polytheism combined with a setting resembling medieval or later Europe Pretty self-explanatory.
Systematised magic That is, the reader is told the ‘laws’ governing magic. Wizards are often analagous to academics in the real world. The Earthsea series is a partial exception, in that it has systematised magic and a school of magic, and dates from the late 60s.
China Mieville’s Bas-Lag trilogy is an example of a post-D&D setting with all these features, and he’s said that D&D was an influence on him.