Dear new authors: PLEASE BE ADVISED.
People are rude. There are many people out there who see no reason to bother with courtesy. You're going to meet those people here. They are going to review you for quick points and say nothing of value. They are going to review you more sincerely but be too busy for courtesy. They are going to be arrogant and have nothing to offer but proof of that fact. They are going to read out of their genre and offer extremely faulty suggestions that make no sense.
The onus is on you to make something of all that.
I realize this is QUITE DIFFICULT for new writers. If you're new, you're probably filled with self-doubt. That's a good thing, as the alternative is to be filled with egotism -- a far worse affliction.
You're going to have to learn how to tell the difference between healthy feedback and unhealthy feedback. There's a big difference, and there's no self-help manual here in the archives to help you out.
Welcome to Writing 101. The things people say to you in reviews here may confuse and perplex you. Even the friendly reviews will offer conflicting suggestions. Be proactive. Figure it out.
Take the hits, and learn how to write. Because there's going to be rudeness EVERYWHERE. Amazon reviewers? Not known for their courtesy. Agents don't have time to explain why they rejected your novel. Pulitzer winners get grilled. No one cares that it took you ten years to write your book, or that it was inspired by personal trials, or that you think your use of adjectives is unique and daring, or that you named all of your characters after your favorite pets.
People are going to be tough. They are going to read your book through their own filters. They are going to hate it, or they are going to love it.
Know who you are. Be stronger than all that, because it's going to get worse.
You have to believe in your ability as an author -- more than anyone. WHICH IS A SLIPPERY SLOPE because you don't want to love your work so much you fail to hear tough advice like "your writing stinks" when that's the best advice you could hear. But you also don't want to be so hesitant about your work that you listen to all the conflicting feedback and lose yourself. Because the feedback is going to be conflicting. Remarkably conflicting. One man's gold is another man's coal.
Somewhere in there you must locate the startling truth that is your very own to share, and get it on the page, and realize that it is destined to be wholly imperfect, as you are, and make a decision about what you want it to say and learn as well as you can how you want to say it, and end the day pleased with yourself for somehow finding the ability in this completely disorienting world to try. And you have to know when to let it go. It will never be perfect.
Have enough respect for your work that you are willing to listen to feedback, but never lose sight of yourself or your vision. Learn how to distinguish what helps your work, and what harms it.
If a John Hamler comes along and tells you that you stink, don't assume he's just being rude. You might stink! No shame in it -- we're all learning. BUT, if he offers you absolutely no justification for his remark, and you can't find a way to seek clarity from others on the topic, go to bed at night without troubling yourself over the fact. The realization that you will stink now and then is not an obstacle. Losing your nerve because someone tells you that you stink without telling you why is far worse. Tossing and turning and wallowing in self-doubt is not writing. Approaching your work honestly and doing what you can with your talent is writing. If the advice you receive accomplishes nothing but crippling self-doubt, you may officially discard it. It has not helped.
{Not to worry! If you stink, there's someone out there who will tell you why.} 
Now, if a John Hamler comes along and says, "Gent, your work stinks. Here's the issue," you may have something. This John Hamler may be abrupt and disinclined to take the time to be courteous, but if he can pinpoint what it is about your work that isn't working, LISTEN TO HIM. He has given you something solid to think about. Specifics is what you want. As you grow as a writer, you'll learn to distinguish between helpful specifics and harmful specifics, but for now, you have something specific. You can build on that.
Remember you don't know John Hamler from anyone, so don't give his feedback too much credence. Just take it in, as a means to learn. See it as perspective, not fact. Remember he can probably see a lot in your work you cannot see, because he didn't write it. That's what you want -- responses beyond yourself. Later you can determine whether or not his is helpful. Don't hear it with your heart. Hear it objectively. He has nothing personal against you. He's responding to what you've put on the page.
Be thankful for the John Hamlers who will give it to you straight, because the world is not only remarkably rude, it is astonishingly disingenuous. Courtesy is mighty fine, but truth is finer. You don't want your hand held; you want facts. Take that however they serve it. It's rarer to be honest than to be nice.
Some people have nothing to offer in a review and have no idea what you are trying to say when you write and will lack the talent to realize their opinion is not objective analysis. Some people will have incredible insight and lack the ability to present it gently. There's a difference.
When you are a new writer, you likely have no barometer to distinguish between the useful writing advice and the poisonous kind. That's on you to cure. Do independent research, read ravenously, write, write, write, and don't take things to heart if they seem adverse to common sense. You will learn as you experience -- and that's what the rude reviewers can offer you -- abundant experience. It's going to sting at first. Figure it out.
Don't wait around for the world to suddenly become polite for you and your book. It's not going to happen. Hike up your breeches and take it on the chin. Learn what you can, and don't stay up all night puzzling over stupidity. If you melt every time someone is an idiot, you're not going to make it out the door. Figure out how to hear your own perspective without losing the ability to listen. It's a tough world out there. Be the rare jewel. If you can't change the world, learn how to live in it. Let it make you better. That's my advice for life.
{I'm half writing this for myself. Mean reviews bring me down, down, down. And I found them extremely confusing as a new writer. I think you have to experience that to grow as a writer, though, because the truth is, when others look at your work -- they throw it into relief. You see your own work with more clarity because your vision is juxtaposed with theirs. So your awareness of your own work, your own style, your own voice, and your own distinct strengths and weaknesses grows. You realize what it is that makes you stand out. Some of the things people try to filter out of your work here are the human imperfections that make it yours. When you can figure that out, you have grown. But that's a whole other story.}
To conclude, even rude reviewers have something to offer! They toughen you up. So shake hands all around, folks. We have them aplenty here. 
