Nothing is more frustrating than authors who query without following submission guidelines. The wrong subject line means your email isn't filtered properly and my response is delayed--sometimes meaning we miss out on ever getting a chance to work together at all. People send their query as an attachment, with no information about the project in the body of the email...and no, I don't click unsolicited links or attachments. Not many agents (read: nearly 0) do.
But the worst of all is when I get a well-crafted query with the right subject line with a good idea...and no pages pasted into the email.
Seeing the writing is so important, and it can remedy many common query faults, at least for me. If the story doesn't quite make sense or it sounds like something I'm seeing too much of, seeing great writing will always, always give me pause and make me reconsider rejecting.
So submission guidelines aren't just because agents are ego-maniacs who demand that you jump through a lot of hoops. We take great care with our query inboxes to keep them organized and responses quick. And, even more importantly, we want to give you as many chances as possible to get our attention: query and pages.