1. "I have a problem with your premise." This is the red flag to end all red flags. I don't care how flimsy the premise is. Every idea has the potential to be a good story. Execution is something else entirely, but if somebody doesn't like your idea: don't listen to them. What they're basically saying is "I am not an Ideal Reader, therefore not your target audience, therefore I am not the right person to critiquing your work." I hate, hate, hate people who think you should be writing for broader audiences than your story is capable of reaching. If you're writing romance, you're writing romance for romance readers. You're not trying to reach hard science fiction readers. Very few people even know what makes a breakout mainstream novel that has high market appeal. If they did, every single book ever written would be Harry Potter. It just doesn't happen. And for somebody to ask you to make that happen is ridiculous and unfair. For the most part, writers are writing to please themselves first, and you don't have to feel bad because your fantasy story is not grabbing the attention of romance readers. It's a freaking fantasy! I had someone in my writing group tell me that she found my psychic dolphin story "ludicrous." She basically said it could never happen. Aside from the fact that that seemed to be missing the point of science fiction and fantasy entirely, it affected her ability to feel basic things about the story like emotional connection to the characters or plot stakes. I went home, incorporated her line edits and punctuation suggestions, because that was all she was capable of giving me aside from telling me "this story doesn't work." She had all sorts of ideas of how she would have done it. That didn't mean they were valuable critique.
2. "This would be better as a ________." I keep hearing that my prose work needs to be graphic novels. I keep hearing that my dinosaur story needs to be for children, not teenagers. I hear all sorts of people say that a chapter book should be a picture book should be a board book. And boy, do they love bludgeoning you to death with what something should be. It's a very popular pastime. The problem with this is that it's not necessarily your job as a writer to find your market niche. It the job of your literary agent and eventually the marketing team that will be part and parcel of your publication contract. Believe me, these people know how to sell books. And once you get to that point, they will tell you what your book needs to be in order to sell. (Or, with the rise of ebooks, it'll be your own damn job anyway.) However, publication is a long way away for a lot of writers. So this kind of critique just wastes your time. The only time you need to implement this is when a literary agent is telling you so. And odds are, if you think your story is a YA novel, you've landed a literary agent that thinks so, too. Granted, knowing your book's level is important. You need to know the difference between a chapter book, a middle grade book, and a YA novel. But that's very basic research, which should take you about five minutes on Google. It's not rocket science. This sort of critique is smoke screening, the same sort of useless ideamongering that's direct kinship to "Oh you're a writer? I've always wanted to be a writer!" The most important thing to remember about critique is that it's someone else's idea about what your writing should be. A lot of ideas from other people about your writing are good and should be implemented. But not all of it is gold. Sometimes you get really stupid ideas from other people. And you need to trust your judgment to know when something is a stupid idea that will wreck your story to its very heart.
3. "You can't do (insert arbitrary story movement here.)" I have been scolded for not having my protagonist show up in the first 10 pages, despite the same thing happening in Harry Potter, David Clement-Davies's Fire-bringer, and even the story of Jesus Christ. I have been chastised for characterizing before I begin the plot, when everybody knows hundreds of books that take the time to characterize for emotional stakes before the plot begins. For every time someone tells me I can't use a trope, I can pull up an article on TvTropes where that trope has been used successfully. Please note that critiques like "This story movement isn't clear" or "This story movement makes no sense" are not the same thing, and are (or can be) valid critiques. (And if you do hear that, you should pay attention; odds are it's pretty important.) This sort of critique springs from "knowledge" of the market. Mysterious, ethereal "knowledge." Its most common form is when someone is published, and someone who knows that person seems to think that their success is due to a specific formula. Therefore, if your novel falls outside that formula, you can't do it and expect to be successful (i.e. published). Another popular form it takes is when someone goes to a writing seminar and listens to literary agents talk about trends or things they're looking for. Believe me, you will hear at every writing session you ever go to, every literary agent you ever encounter, that "the writing should be good, the characters should leap off the page, the plot should be engaging." That does not mean that if your YA novel is not a paranormal romance it's doomed to failure. Or that this arbitrary thing you've done will doom you for all eternity. (One of the biggest lessons you learn from popular literature is that "good" writing is not the same as "marketable" writing.) It never seems to occur to these people that one literary agent's opinion is just one literary agent's opinion. Or that nobody sees bestsellers coming, and that one book's success becomes the next round of "You can't do (insert arbitrary story movement here because (bestseller) didn't!"
4. "______ is too scary/controversial/morally wrong and shame on you for writing it!" This one is a double-edged sword that lurks in children's writing groups and among literary agents. Everybody's out to sell books, and for some reason it never seems to occur to these naysayers that controversy sells just as well as good wholesome morality (or better). The younger you go, the more scrutiny you're under to be politically correct, but I've seen people who write YA scolded for having sexuality in their books and, no joke, a children's picture book where a dog chases off thieves being scolded for "too much real-life fear for a child." When writing becomes a social event, it can be a lot of pressure on a writer given to the demands of the audience. Regardless of whether it's your target audience or not. I've seen entire stories wrecked by this. The climax is too controversial, so every single building block of the story leading up to it gets demolished. And more often than not, the story gets destroyed in the process. It just becomes this bland, happy, generic thing. This rubs me the wrong way as an artist, and as somebody who feels of the role of the artist is to be controversial. To forge new paths and make new statements. Given the "Hollywood" trends of a lot of YA literature, we need more original, breakout stuff with profound and real things to say. Not less of it. I can freely admit that a lot of people write for the express purpose or being published, not to make an artistic statement. But I maintain that this attitude is an insidious one. I believe it has a direct correlation with the cookie-cutter YA that keeps telling the same story over and over and over, in ever narrowing margins. I'm not published yet, but if that were the sort of feedback I got from a literary agent, I would not implement it. I just don't think it's valid critique to be told "you can't" rather than "this is done well despite the controversy and stands on its own merits despite breaking from the norms."
5. "Well, I don't read (genre), but I don't like (obvious genre element) and think you should cut it." This gets back to the Ideal Reader thing. I can't stress how much a difference it makes to have an Ideal Reader. And Ideal Reader is not somebody that will tell you "I love everything you write!" An Ideal Reader is more than likely a member of your target audience. I have had my fantasy worlds scolded for being too detailed. What I think is an intriguing and interesting detail, my writing group often accuses me of world-builder's disease. Each is its own battle, and sometimes they make good points, but nonetheless I balance every bit of critique I receive with the knowledge of fantasy that I like to read. But when somebody starts telling me they don't like magic or psychic dolphins, that kind of critique is going to go in one ear and out the other for me. I read fantasy. I read a lot of fantasy; I read a lot of types of fantasy. I read fantasy on different reading levels. I read myths and fairytales and have studied deconstructions of myths and fairytales. So when someone who's never read a fantasy novel in their life starts telling me how to handle my fantasy, I don't have to listen to that. I have the right to trust my own judgment when it comes to an element about the genre I adore. Again, please note that things like "This element isn't clear" or "This element doesn't make sense", that is not the same criticism. This is when someone tells you to remove your psychic dolphins from your psychic dolphin story because their premise is too ludicrous. This is someone telling you your romance novel has too much romance in it and could you ease up on the steamy scenes already? This is a grey area for sure, when someone starts telling you to remove the part of your genre that makes it your genre, that's another pretty big red flag. You need to put on the brakes and examine where this person is coming from before you start to doubt yourself.