This post is specific to the dangers and courtesies of asking for feedback on a manuscript that is not complete. (I address the dangers of letting others critique your completed first draft HERE.)
The danger of asking for feedback on an incomplete / unedited manuscript is this:
The most dangerous out of this list is this: asking for feedback on an unfinished manuscript is DELAYING YOU FROM FINISHING THE 1ST DRAFT!!!!! I used every formatting button available to make that stand out. Now read it again. If you ask for feedback on an incomplete manuscript, it means you have not finished your first draft. You're instead stuck in an endless loop of feedback-rewrite-feedback-rewrite-feedback-rewrite-feedback-rewrite-feedback-rewrite-feedback-rewrite-feedback-rewrite-feedback-rewrite-feedback-rewrite-feedback-rewrite-feedback-rewrite-feedback-rewrite need I continue? You're stuck in this loop and never finish the novel. And isn't the point to finish the novel?
You teach yourself not to figure out problems for yourself and rely on other people to give you answers. This is dangerous because not everyone has good advice, and if you haven't trained yourself to know the difference, you could be making changes to your material that lessens the quality.
You'll be discouraged because 1st drafts, complete or incomplete, are crap and just because you think it's cool NO ONE ELSE WILL!!!! Again, that I-used-every-formatting-button thing. Your writing community will say that it's crap (nicely, of course), and that will discourage you, and you'll stop writing. And you don't want to be discouraged and stop writing.
***Disclaimer: some writers have become offended when they've read the rest of this article. If you are pre-disposed to offense, don't read further. Also, if you don't think any of this applies to you, then it doesn't apply.***
Hold yourself to a standard. If you don't hold yourself to a standard, I'm suspicious whether you want advice or want someone else to write the story for you without giving them the courtesy of paying them for their time and creativity.
A writer in a writing forum dumped his prologue and first 2 chapters to the group to read and critique. His disclaimer said this: "Do not be gentle, but also ignore grammatical issues because nothing has been edited haha."
I was stricken by this, exactly like someone putting their trash at my feet and asking me to take it out for them. I had this sense that he didn't want to use his own brain power to figure out his own problems with his writing, but, in the un-edited form, he wanted to throw it others so they could feed him the answers he himself didn't want to think through on his own. I matter. My energy matters. So I declined.
Don't give your incomplete / un-edited material to others and ask for feedback. If you're not willing to put in the time and energy, then you offend those whose time and energy you ask to spend on your behalf. You put in the time and energy first, then I'll match it with mine. Exactly like the 401K.
Back when I WAS reviewing 1st drafts for new writers, I unintentionally made writers feel bad when I pointed out what was wrong with their work instead of praising them. I can't even say this was 1 or 2 cases. This has been every single one, like 20.
What was happening is a writer would give me their first draft, I pointed out the "faults", and the writer responded super disheartened over my review, or they would never contact me again. I realized I was hurting them more than helping, which is why I no longer review 1st drafts and why I wrote this blog post. I care about all writers and their writing so much that I don't want someone like me reviewing their 1st drafts and disheartening them over what they might say. 1st drafts are always trash: mine, Stephen King's, everyone's, and it's not fair for anyone to receive criticism on a 1st draft when they likely would have fixed those problems come drafts 2 and 3. I don't want anyone to get disheartened and stop writing because someone points out faults in their draft. I've seen it happen multiple times.
Follow my tried-and-true advice if not for the sake of your fellow writers, then for yourself, because you and your writing matter, too.