Amanda Valentine talks about self-published work and some problems with final product quality she's encountered:
Here’s the thing, though—the writing wasn’t bad, at least not irredeemably so. The book just wasn’t finished. It was someone’s first draft. Sure, it was probably proofread by a few helpful people, but it was obviously never seriously edited. What I was reading was a proofread first draft, not a final product. For example, each character was introduced with a paragraph that sounded like it came from an outline—she looks like this, her interests are this, her strengths are this—and each brought the plot to an uncomfortable halt. Combine that with a painful abuse of commas and some uninspired dialogue, and a couple chapters in I just gave up
She's right: a good editor (hell, even a mediocre one) contributes significantly to a story. I'm not even going to list the ways, in case an editor yells at me for leaving one out. Amanda provides some examples of collaborative improvement in writing projects that she's encountered.
Personal experience with editors compels me to tell you: read this story in full. (Here: http://www.ayvalentine.com/2012/04/respect-your-readers/)