I've been having an interesting conversation with another editor -- a highly skilled, very bright editor -- about when not to edit something. Editing isn't about changing everything. Editing is about choosing which things to change.
We were talking specifically about line edits, so I thought it might be interesting to look at some of the things I am least likely to change in line edits.
I rarely change an author's paragraphing. If I change paragraphing five times in an entire novella, that's a lot. Any such changes are generally meant to separate actions and reactions to create more impact. Even with those kinds of changes, though, I've been known to let it alone if the author's voice supports that kind of long, run-together paragraph. For some authors, this is an important part of voice.
That's not to say that I don't often want to change paragraphing. Just that the urge strikes much more often than the change is made.
If an existing paragraph ends on a weak note or buries high-impact elements in the middle, I will reorganize the paragraph's guts before I will change the paragraph breaks.
I leave alone word choices except in three cases.
First, if the word is misused. I never edit without dictionaries at hand. I check even familiar words if they're used in a way that isn't rock solid. Sometimes, using a familiar word in an unexpected way can be a sign of greatness. One of my authors recently surprised me with several very creative uses for common (but vivid) verbs, and because meaning was clear and the result was dramatic and interesting, we didn't pull those unusual usages.
Second, if the word is overused. This requires a bit of judgment. Unusual words stick out a little bit, so you can't repeat them as often. Even common words can really get annoying, though, if they're overused. There are only so many words you can use to describe the fact that a person looks at another person. In cases like that, it's not just about word choice but about action choice, so we might pull the whole action instead of just doing a word substitution.
Third, in very rare occasions, a writer will use a word that's out of voice or out of context. This is more likely to occur in stories with heavy world-building such as historicals or futuristics. If a medieval Scottish warlord asks his vassal, "Are ye on crack?", we have to change it. These kinds of things don't come up too often, but when they do, they're memorable. I have a great war story about spending an hour investigating the history of self-striking matches to determine when the phrase "struck a match" might have come into usage.
But other than these three situations, I generally leave an author's word choices alone. Even if I can think of ten synonyms that I like better, I leave them alone. It's not about which words I personally like or don't like. It's about making this author shine in a powerful story.
I don't often change what's inside the quotes. If I do, it's usually the punctuation and not the language. I might, for example, shorten up the dialogue beats within a speech by adding a period or a comma. And if there's a semicolon, it comes out. Ditto for colons. But the bottom line is that dialogue is easy to write to a certain level of competency (and very hard to write brilliantly), so there's almost never a need to change it.
There are exceptions, such as when a character says something shocking or provocative and it would be unnatural for the other characters not to react. Then we might edit the dialogue to make the most of that shocking detail. Or if a character needs a heaping helping of STFU, we might trim it back. Very rarely, we have a male character who sounds female ("In my heart, I know we tried our best, okay?"). Then we just butch him up, which usually means stripping out the question marks, feeling words, and anything cheerleaderish.
So there you have it. The three things I'm least likely to edit, and the few things which will motivate me to change them.