There is always some structural editing. One may have polished a manuscript to flawless perfection, but that is largely irrelevant to the process, for two reasons:
First, the author's definition of perfection probably has something to do with the quality and integrity of the work; the editor's definition is as likely to be focused on marketability. Sure, that allusion is brilliant, funny, and exactly what that character would say in that moment, but it's over the heads of the mass-market audience and therefore a threat to future sales. It is the editor's job to raise the possibility of making the work more accessible. Yes, the hero needs to die tragically in the last scene -- but that decision precludes a sequel, and marketing costs could be better amortized over a trilogy—or even better, a series -- than a stand-alone novel. And American audiences in particular, prefer happy endings.
Which is, I hasten to clarify, not to suggest that all editors are philistines—quite the contrary in my experience. Just that their job is to help the author reach as large an audience as possible, and that minor adjustments can sometimes make a big difference in sales. There is probably no threat to the integrity of the work in changing "boot" to "trunk" and "torch" to "flashlight", to sell a British author to American audiences; but authors may justifiably balk at changing a gay character to straight, or a black character to white to pander to the prejudices of the lowest common denominator of the American mass market.
Authors may have lots of horror stories of changes demanded by editors [my personal favorite is the Canadian screenwriter who was asked if he would mind adapting his biography of a serial killer into a musical]; but for every author story, I can cite ten even more ridiculous accounts of authors unwilling to change a single word or comma in their far-from-perfect manuscripts. Authors, by definition, are too close to their own work to be able to spot the flaws in their manuscript. Development editors approach the manuscript with fresh eyes, and easily spot the lapses in logic, the sudden slowdowns in pacing, the out-of-character actions, and so on, that an author cannot. In the vast majority of cases, the editor is correct when identifying problematic areas of the current draft, though there may well be alternatives to their suggested fixes. Every suggested change is likely negotiable, but the author has to be willing to change—or to walk away from the deal.
It is true that not all editors are equally good, or equally appropriate for any given manuscript; finding the right editor to partner with is an important element in the successful development of any book. If one starts from the assumption, however, that the editor is the enemy and all their suggestions 'tampering', then the potential benefits of a successful working partnership are at risk, and the work likely impoverished thereby. Instead, start from the assumption that one's manuscript -- like all manuscripts -- could benefit from a second set of eyes, and that the editor appointed by the publisher is the most suitable for manuscripts of that sort. The careers of everyone involved is dependent on getting it right, so they really are trying to help; and not part of an international conspiracy to block or undermine new authors.
This column originally appeared in Writer-in-Residence: Common Sense Guidance for Writers by Writers, curated by Krista D. Ball http://writer-in-residence.blogspot.com/2011/03/editor-is-not-your-enemy-part-2.html