For example, before reading the author's notes on HPMoR I was not familiar with Chekhov's Gun.

Really? I remember being about eight years old, watching an episode of Power Rangers, and seeing some random no-name tourist drop a camera and the shot lingered on it for about half a second. My instant thought was "Oh, the bad guy is going to be built from a camera", and sure enough it was. They'd never have put that in the episode otherwise, and that was obvious to me at that age.