Is having a good idea for a middle grade novel enough in publishing? What I'm trying to find out is at what point is a manuscript considered to be too much work? What does "too much work" mean from an editor's perspective? Do you ever encounter situations in which your love of a book makes the "too much work" aspect not as important and insurmountable?I'm not absolutely sure how to answer this question. If you mean, is having a good idea enough to excuse a phenomenal lack of writing ability, the answer is no.
As I've mentioned before, ideas are not everything. If everything about the writing needs work, then it's almost certainly too much work for any editor, no matter how good the idea.
That said, there is wide variety in types of writing strengths. Some writers are great at dialogue; some at setting; some at character; some at plotting; some at mood. The best writers are good at pretty much all of these, but if your writing is hitting just one of these qualities in a way that is particularly compelling to the book's audience, then that may in fact be enough. Maybe.
Every editor weighs how much she likes a manuscript against how much work she thinks it will be to edit it. Do you have a bad habit of adding adverbs to all your dialogue tags? That's irritating, but it's also EASILY FIXED. Do you simply have no idea how to show, not tell, and your fantastically plotted novel is by turns condescending and boring because of it? That's irritating, and ALMOST IMPOSSIBLE TO FIX.
And there's also the question of the author's ability to receive one correction and run with it. I've critiqued first pages that had a pernicious fault, like overuse of adjectives. The author thanked me kindly for my input and asked if she could send me the whole manuscript. Sure, ok. When I got the manuscript, that fault had been fixed on the first page AND NOWHERE ELSE. People like this are a long, long slog to edit.
One of the misconceptions editors meet most often among non-editors is that we can fix anything. No, we can't. We don't have superpowers. Our ability to make anything better depends on our ability to convey to the author what's working and what's not working and why, and the author's ability to apply that knowledge. In order to edit anything, it has to be the author who fixes things, not us.
If an author simply can't write, well, no one can fix that.