1. Query contains typos, misspellings, grammatical and punctuation mistakes. Yes, people actually send out such queries. This creates the impression that the author is not a professional. It makes agents suspect that the manuscript is full of mistakes, too. Take special care to make sure your query is perfect in these terms before you send it out.
2. Query lacks basic information. Agents are extremely busy individuals. They want to know three things when they scan a query letter, and they want to know them in a hurry: a) what your book is about, b) who you are, and c) why you have queried them (as opposed to other agents). If you omit any of these basics, you risk the agent moving on to the next letter.
3. Query contains irrelevant information. This is the opposite of (2). Agents assume that if your query contains information that's irrelevant to the book you're presenting to them (such as your education, job, hobbies, travels, family relations, your pet parakeet, etc.) you're not capable of writing a focused story. Good stories can only be written by authors who know what to put in and what to leave out. So, if you spent ten years working in a police forensics lab and your book is a detective novel, by all means, include that in your query. However, if your book is a romance with no murder subplot, leave it out.
4. Query fails to state genre of book. With some thought, every book can be classified under a story genre—romance, mystery, children's middle grade, fantasy, etc. Even novels loosely labeled as "literary fiction" can still be classified under a story genre—one merely needs to identify the main plot thread to narrow it down. Catcher in the Rye can be said to be a coming-of-age story. Tess of the d'Urbervilles could be a romantic tragedy. You must do the same with your book--give agents a "handle" to grasp it by, even if the handle doesn't seem like a perfect fit (it probably won't be). Otherwise, these people will not know which shelf it will go on in the bookstore or how to talk about it to publishers. (Sadly, this is why a lot of wonderful fiction is never sold, but as the old saying goes, you can't fight City Hall--self-publishing is the best alternative).
5. Query fails to differentiate your book from other similar-sounding books. Just as agents want to know on which bookstore shelf your novel will appear, they also want to make sure that it will fit in between the books of two known authors, not on top of those authors' books It's important to remember that agents are seeking original writers, not copycats. If your book sounds remotely similar to any other books that have ever been published (and you should do your homework researching this), then you must explain why yours is different.
6. Query fails to show how your book is similar to other books. This may sound like a contradiction to (5), and, in a way it is. If your book sounds like something that's so "out there" that they can't get a grasp on what it is, this can be problematic. The bottom line is, agents want to be assured you have something that's different…but not so different that distributors and retailers don't know what to do with it.
7. Query fails to identify basic character goals & conflict. Every good story is born of conflict between a character wanting something and another character stopping him or her from having that something, even if, in the extreme, the second character is "inner guilt" or "nature." Don't expect agents to pick this up in between the lines—come out and state it directly. Who is your hero? What does he/she want? Who is your villain? How is he stopping the hero from getting what she wants? The simpler and clearer you are about these basics, the more quickly and easily agents can grasp your story concept and ask you to send more.
8. Query is written in too flat a tone. If your story is light and humorous, so should be your query. If your story is dark and mysterious, ditto. Your query should match the style of your book and demonstrate your writing proficiency. Don't miss the opportunity to flex your literary muscles.
9. Query lacks creativity/fails to excite. This problem often arises from following a query-writing formula and/or a lack of confidence on the part of the writer. Formulas are useful for learning the basics of query-writing, but if you want to get the attention of agents, it's a great help if your query is different in some way. You want to turn them on, to whet their appetites, to make them hungry for more! Let yourself be excited about your book and allow your enthusiasm to come through in your query (however, see the next item below).
10. Query reads like an advertisement. As the author of your novel, you should be excited about it, but you must stop short of praising your work yourself, and especially in the vein of a dazzled book reviewer. You have no objectivity whatsoever about its basic literary quality, and agents are woefully aware of this. (This is also why self-publishing and self-promoting are extremely challenging with fiction). You must show the agents that you have a wonderful book, rather than tell them. This is done describing the story in a way that makes them think, "Hm...that sounds like it could be a page-turner..." But don't use these kinds of adjectives to describe your book yourself. Also, including any praise from family and friends is a waste of time and will make you look like an amateur--resist the temptation.
11. Query boasts of book's overall market potential. Agents assume (and usually correctly) that you are not an expert on the publishing business. They know far better than you whether your book has bestseller potential, movie potential, merchandising potential, etc. Let them decide these things for themselves.
12. Query is too long. No book has ever been written that could not be well-presented with a one page query. Remember that the query letter is only a short pitch to entice the agent into reading some sample chapters, or, in the best case, your entire manuscript. If you're absolutely sure that you can't fit your query onto one page (without any tricks like changing the margins or font size), trust me--you can. (See my example in 5 Steps to Landing a Good Literary Agent) The best queries are like Hollywood "elevator pitches" for movie ideas-- they are short and focused, so tight that the could be delivered to an agent you might run into on an elevator in 60 seconds, with the result being "Hey, that sounds pretty good -- why don't you send me the first three chapters?"
13. Included synopsis is too long/detailed. This is usually the reason that the query letter itself becomes too long. Again, remember a query is just a pitch for the book, not the book! In your short synopsis, bogging the agent down with details like your character bios, back story information, and subplots is a likely way for your query to be passed over. Stick to the main points of the story--the major characters, their goals, their conflicts. Read other short synopses of movies and books to get the feel of an appropriate length and level of detail.
14. Query includes rejection history. Agents do not want to hear about how much trouble you've had selling your books or how hard it's been for you to entice agents and editors to read them. Even telling agents about one former rejection is a mistake. They want to believe they have stumbled upon an impressive new player on the literary scene that their competitors have not had a chance to discover.
15. Query is written in too informal or chummy a tone. Agents are professionals and expect to be treated as such. If you don't know them and they don't know you, they expect a certain formality during the initial contact. This doesn't mean that you shouldn't use humor or sarcasm or even four letter words in the body of your query, if appropriate to the style and tone of your book. It simply means that you should adopt a respectful posture when addressing the agent directly, the same tone you would use to speak to any professional with whom you have not yet established a business relationship.
In summary, the above 15 mistakes are general ones--there are of course other mistakes made in query letters. But based on my own experience and that of working with other writers, these 15 seem to be the most common. If your query letter is not generating the response you would like, go back through this list to try to figure out where the shortcomings might be.
If you can't seem to pin down the problem, let a few trusted friends read your query. With their greater objectivity, they might be able to help you fine-tune it.