He says it's this: Not trusting the reader to get it. As in, too much set-up, too much backstory, too much explanation.
I think he's right.
But to say that writing such material is itself the Biggest Mistake is to fail to dig deeply enough. Why do we pile this stuff in? Maybe, to some degree, it's because we simply like the material. We made it up, and we're bound and determined to cork it into the narrative. But this reason pales beside the Biggest Reason: We don't think readers can follow the story unless we include it. We don't trust their intelligence. We don't trust them to get it. We're writing down to them.
If we really think we can't trust them to get it, remembering the last movie, play, or TV show we watched should change that thinking. Through these media, stories unfold before us in visible and audible scenes, and we simply have to process what's going on and fill in our own holes. "A long time ago in a galaxy, far, far, away" notwithstanding, movies don't give us backstory before they get rolling. They just roll. And most audiences get it. Because other entertainment media compete with written fiction like never before, more and more movie techniques are used by the story writer or novelist. We can't overload a story's beginning with material meant to "orient" the reader any more than a movie maker or playwright can. Not even if we disguise it as a prologue. We must provide what is needed for the reader to understand, yes. But these are carefully chosen details, actions, and bits of dialogue that work within a present, ongoing scene. Not an info dump.
Edgerton posed the question "What's the single biggest mistake writers make in their story beginnings?" to several editors and agents. Here are the replies he got:
- Thinking it's okay that the story starts slow because it "gets better later on."
- Writing a beginning that's all backstory.
- Baiting and switching -- writing an exciting beginning that doesn't relate to the story.
- Assuming they don't have to earn the reader's interest.
- Beginning with a dream, the weather, or background info.
- "Front-loading information."
- Starting with backstory or character descriptions.