I got this off of the blog of literary agent, Rachelle Gardner. There's more information on there worth checking out that I didn't include. This entire article is from her POV.
1. Not making me feel special.
Multiple agents are listed in the "To" field of the email.
2. Not knowing or caring who I am.
Your letter is addressed "To Whom it May Concern" or "Dear Agent" or to another agent, or with no salutation at all.
3. Making me answer "no" after one sentence.
Your query begins with a rhetorical question. The problem with this is that usually my answer to your question is "no" so you've already lost me. Especially the "Have you ever wondered...?" questions.
4. Putting the cart before the horse.
Your query begins with "This is the first in my planned 9-book series." Don't do this! Pitch ONE BOOK first. Toward the end of your query, you may include a brief sentence something like, "If this book is successful, it could easily become a series." Another getting-ahead-of-yourself mistake is talking about the awesome movie your book will make.
5. Fudging the truth.
False personalizing: Pretending you have a connection with me when you don't. Or false referrals: Saying somebody referred you when they didn't. It drives me crazy how many people write, "Michael Hyatt recommended you." When what they really mean is: "I found a list of literary agents on Michael Hyatt's website, and you were on the list."
6. Fudging the truth, part 2.
Saying you're a previously published author when what you really mean is that you self-published.
7. Intentionally breaking the rules.
You acknowledge that I don't rep a certain genre or category, but you're pitching it anyway.
8. Being stuck on yourself.
Your query is 90% about yourself, 10% about your book. I need to know about the book! Especially for fiction. For non-fiction, since platform and qualifications are so important, your query can be 60% about the book, 40% about your platform.
9. Making it obvious you're not a good writer.
Your query is poorly written with bad grammar and punctuation, poor choice of words, lousy sentence structure, no unique voice... showing me very clearly that you can't write.
10. Ignoring my submission guidelines.
I ask that you include the word "Query" in your subject line, and that you include a few sample pages of your manuscript, pasted into the email. I also ask that you do not include attachments or expect me to click on links. It's not that difficult.