Okay, some demotivational columns worth reading:
The funny thing about being a development editor is that I can divide most of the authors that cross my desk into one of two categories: highly talented writers who are too shy and (unrealistically) dissatisfied with their work to seek publication (and who need a writing coach mostly for moral support), and aspiring writers who have no idea how bad their work is (who need a writing coach to tell them to go back to the drawing board and never ever let the current manuscript see the light of day). The former are a lot easier to work with, but the latter often do quite well if they can put their egos aside long enough to take the advice they're given; and if they are willing to undertake repeated revisions, not just first drafts. Sadly, many in the second category just shoot the messenger and fail to make any forward motion. Fortunately, most of those will want to design their own covers too, and so effectively warn buyers away from their work (see previous post).
I'm egotistical enough to think I can help any writer get better...but I do occasionally tell writers that they are not sufficiently close to their goal that paying for developmental editing would not be cost effective for them; that they need to go back a couple of steps: perhaps take some writing courses, join a writers' workshop, write two or three other novels, etc, before trying to bring the current manuscript up to publishable standards.
And sometimes our styles/tastes/issues are just too incompatible to be workable. A good development editor doesn't take on clients s/he cannot help, and that is more often about being the wrong genre than about the quality of the writing.
So, I can usually help most SF writers get better. But it helps a lot if the writer starts with a little humility. Thus the demotivational references above. Get a little perspective before demanding that others love your manuscript....
Oh, but don't read those columns if you fall into the first group; that is, someone who worries that their manuscript is not good enough. If you are worried that no one will want to read your book, these guys aren't talking to/about you.