My own experience with editors and copyeditors is almost universally good. I've had one bad copyeditor experience, on a piece of short fiction, which was speedily resolved by an appeal to the editor of the magazine in question. (NB: a copyeditor is welcome to fix my word repetition, crappy writing, to query my factual errors or inconsistencies (eye color creep is an infamous case), to repair grammatical wackery and malapropisms, and generally do her job. She's not empowered to restructure an entire short story. And if she gets snippy with me when I answer her queries with factual references, photographs, and footnotes, I will tell her boss on her. That said, copyeditors are among the great unsung heroes of writing, and I know I owe, at the very least, Paul Witcover and Faren Bachelis a very nice dinner apiece for keeping me from going out with my skirt tucked in my knickers. And there are other CEs I would praise as well, if I but knew their names. (Which is not to denigrate the contributions of the other unsung heroes of publishing--a good book designer, art director, publicist, and sales staff are all worth their weight in gold, and independent booksellers are an essay unto themselves. But this is not that essay.))
A good editor has his or her own style, and every editor has strengths and weaknesses (like every novelist)--but a good one will tease out the heart of a story, identify inconsistencies and the hesitation marks and bracketing shots that creep in while the writer is sighting in on the target and wrestling the mighty anacondas of theme and plot. He'll identify the bits where the writer is thrashing, or masturbating, or explaining the story to himself (we all do it--books are often too big to fit neatly in one's skull all at once), and also the bits where something seemed so obvious to the writer that it needed no explanation, whereas to the reader it's an elephant in a dark room. She'll settle infelicities and bring the writer's style to the fore.
In extreme cases, she may indeed rewrite entire passages. Sometimes there are reasons for this choice.
I've got my fingers crossed that I never have the experience to write a similarly discursive essay on what a bad editor is like.