You don't have to be from the planet Krypton to have X-ray vision. I mean, it probably can't hurt if you are, either, but it's not a necessity.
In fact, as readers, we writers have very well-developed X-ray vision, when it comes to others' manuscripts. You can read someone's work and go “Hey, you're kind of a misogynist, aren't you?” or “You must be from Stony Brook, since every single book you write is set there.” and you're probably correct.
The issue comes when we have to do it to ourselves. It's kind of like psychoanalyzing yourself—it's not easy, and all that denial is going to make it even harder. And you will deny unpleasant things, when you find them, but if your reader feels like you're preaching at them about something, even if you do get published, no one will like your book, and that's not good.
So you sit down with your manuscript and you realize, in shock, that you've been subtly hinting at the stupidity of menial labor from word one—of course, it doesn't seem quite so subtle now that you've had it thrust in your face. But what the hell are you supposed to do? Dissect your entire manuscript and fix this previously invisible preaching?
You, the narrator, has a duty to be a bit—just a bit—impartial about your world. Now, if you have a character that thinks menial labor, Christianity, quantum physics, or the cell phone is stupid, that's fine, maybe even two characters (although that's kind of pushing it, in my opinion), but all of your characters can't possibly hate the exact same things—that's preachy, annoying, and thoroughly unbelievable.
How the hell are you supposed to fix all of that? Luckily, it's not terribly difficult, usually, just time-consuming. See, this kind of thing isn't really hidden, it's just hard for the writer to see. It's actually, more often than not, a kind of superficial fix, kind of like a snake shedding skin. You remove the offending surface from the entire body, possibly leaving a few little bits—they don' really hurt if they're diluted—and voila!
Will it make your manuscript better? I think so. Of course, if the entire point in writing the book was to preach about one thing or another, then I guess you maybe shouldn't take all of that out of the manuscript, but barring that it kind of needs to be fixed, otherwise you have an issue.
So now I scurry away to write something...not sure what, but here I go.