And I can understand where it comes from, to a degree. Yes, agents have ideas about how to pitch things. And they have ideas about what is popular. And they have ideas about what sounds like good writing. And for heaven's sake, yes, put two agents in a room together and they may disagree on all but one thing that they both prefer to see. (Want evidence of that, mosey on over to WriteOnCon and visit the live chat transcripts from this last year's conference.) But all those things they say work? They're things they've seen work. When you view 100 queries a day—and I can't fathom that, the most I ever saw as the slushpile reader was ten!—you have a sense of what is going to make an individual query stand out in those 100. They aren't "rules." As someone who sat on first one side of the desk and is now sitting in a tiny kid's chair on the other, as near as I can tell, there is only one rule:
Write an awesome and marketable book.
Take the time to pitch it well.
And then, be ready to keep pitching, and to keep revising, and to keep working.
That's all. Yeah, you can get caught up in write the query this way, write the query that way, don't use a penname, do use a penname, don't ever send an email with an attachment, don't double-space in an e-mail, do double space in an email, yadda yadda. But in the end, I've found that it's more about just using common sense, and being professional, and first and foremost, having a good book.
Of course, I'm not agented yet, so, take that with the requisite canister of salt. But in the meantime, I'm holding fast to a great piece of advice I stumbled across just as I was starting on this whole adventure (and I wish I could remember the source, but I don't--if someone knows, please help!):
There's a term for writers who don't give up: Published.