The third is a slightly different set of advice given out to writers: compare your work to that of established authors in your genre. I've heard a lot of reasons for this, from "it proves you know the market" to it giving the agent an idea of your book's tone. I've also heard a great many agents say it's a horrible idea. Do you have an opinion on this?
Oh G-d, yes. We see this all the time, and while I suppose there might be a way to make it not harm your query, I've yet to see that. To me it's just a technique for spotting what I call an "overedited query." While it's not necessarily a bad thing that writers do research on what a query letter is supposed to be (in fact, it's a very good thing), it becomes obvious after a while that they're pulling out every trick they read on every website to sell me their manuscript, when in fact, all I really care about is if the hook is well-written and makes the book sound interesting. The only thing we care about is your writing. Listing writing credentials is only a plus because it proves to us that other people have seen your writing and assessed that it is quality.
Do not compare yourself to a bestselling author and/or literary genius. You are not currently a bestselling author and time has yet to determine whether you are a literary genius. Also, the other reason for comparing your work to the writing of other authors is bogus when you think about it. Someone will say, "I've written a legal thriller like the works of John Grisham." Look, if you've written a legal thriller and it's good, it will be read by people who like legal thrillers, so chances are it will be read by people who like John Grisham. That doesn't mean you're him. It doesn't sell your book to us; it just makes you seem presumptous about your writing. If your hook invokes in us the feeling that, "Wow, this is just like a John Grisham novel - it'll sell a million copies!" then you've done your Grisham-related job. His name didn't have to be mentioned.
Don't waste space, don't waste words, and don't waste our time. That's all we ask. Oh, and include an SASE if you actually want a response.