How to Avoid an Auto-Reject

From the blogs I've read, there are some things that will automatically count against you. For example, obviously sending a query letter out in a mass-mailing to every available agent. Other things are self defeating, like a query letter that only gives your website address and states that the agent must go there to view your synopsis and sample chapters. Agents are busy. They aren't going to bother.

Other things will get you automatically rejected, like sending a query letter to an agent who doesn't represent your genre. This happens all the time, partly due to bad information. For example, I've done a "fantasy genre" search on AgentQuery.com and had some agents listed in the search results that clearly state that they don't represent fantasy. Always double-check that the agent represents your genre before sending them a query letter. Go to their website and check, if they have one.

Also make sure that you know about how long a book in your genre should be. If it's an adult fantasy novel and has only 20,000 words or a whopping 400,000 words, it will be rejected. Not only would an agent have difficulties finding someone to buy a book outside of the normal word count range, but it shows that you didn't bother to find out what was expected in your genre.

Make sure to talk about your story in your query letter. Agents tell stories of query letters that talk about what a great writer the person is and lists their writing credits, but which forget to explain what the story is about.

Agents have said that don't like it when authors compare their book to a best seller. Some agents like you to tell them what books are like yours, but comparing it to a best seller or saying that it will be a best seller just seems laughable to them.

Agents have also said that they're turned off by people who don't write their query letter in a professional manner. They aren't interested in working with someone who bashes on their own book, or who pleads with the agent/editor to publish them, or who acts very arrogant in the query letter.

They also don't like gimmicks in query letters, like writing the query letter from the characters point of view. Query letters are business letters, so treat them like a business letter.

According to The Rejecter, assistant at a literary agency, here are some more reasons why query letters are rejected:
A Typical Day's Mail
Bad Query Advice: Discussing Your Audience

Evil Editor also has pointers on What Not to Put in Your Query Letter.

On the other hand, Agent Pam Claughton talks about what you should put into the query letter:
Demystifying the Query Letter

Hope this information has helped someone.