What common pitfalls do self-published authors face?

I agree with Shawn Bell, except that I wouldn’t call it “poor editing”. I’d call it “no editing”.

It’s so easy now to write a book and throw it up on Amazon that anyone who has an idea for a story can do it. The idea may be a good one but I’ve seen self-published books written by people who could tell a good story but could not spell, punctuate, or construct a grammatical sentence. Those are just the basics.

Then there’s clunky writing. Long expository passages a/k/a info dumps, too many adjectives and adverbs used when a strong verb would be better. Example: “He ran quickly to grab the fire extinguisher” as opposed to “He sprinted to grab the fire extinguisher.”

From some of the questions I see here on Quora, it’s now not uncommon for someone who has never written anything but school assignments, never worked at improving their writing, never read books, articles or blogs about writing, and perhaps never read much at all to decide to write and publish a novel. Their final product would probably be on a par with what I could achieve if I decided to build a house, thinking all I needed was some lumber, maybe some concrete, and a few tools.

The most common pitfall in both of those cases is lack of knowledge and preparation.