My brother asked me once why my photos of his kid are better than his photos. I said, "Editing." Sure, I have a fancier camera, and a better understanding of basic photographic principles. Still, I think editing plays a huge role in getting a better finished piece. And it starts in camera. Framing a photo, deciding on aperture and lenses, all of it is mentally editing. Taking out anything that doesn't work, elements that distract from a photo instead of adding to it. Deliberately drawing focus to certain features, and away from others. Choosing the best angle. You take out the things that don't work, and when there's nothing to take away without sacrificing your goal, or your message, then it's right. Mental editing.
Once I start going through photos, I edit them down even more. Out of focus? Gone. Bad angle? Deleted. I sat in on a talk with Jared Platt, and some of the other photographers attending were shocked at how he could so heartlessly delete photos of his own kid. "But that one's cute!" Cute, maybe, but the focus was off.
I think a certain level of detachment is needed when you're editing, and I think this is something that I picked up from having a background in design. We routinely did class critiques on our work, and you couldn't afford to take comments personally. Either something worked, or it didn't. If it doesn't serve a purpose, then why is it there? It doesn't matter how long you spent on it, or what you had to go through to get that shot. If the end result isn't working, none of that matters. Because your clients aren't seeing the effort - they don't know the story behind it. All they see is the end result.