These are the very few things that make me stop reading:
1. The query isn't from the author. That includes query writing services, cover letters from your assistant, free-lance editor, etc. You have to write and send your own query letter.
2. Misused words. I don't mean typos. Mistakes happen. I mean misused: allay for ally; accept for except; cash for cache; bate/bait (as in baited breath, something that always makes me laugh); flout/flaunt; past/passed; reign/rein; sight/site/cite; slough/slew.
3. "fiction novel" used to describe a novel. I don't mean women's fiction novel, or science fiction novel, only "fiction novel." All novels are fiction.
4. The query is sent as an attachment. I don't open those.
5. The query letter includes a diatribe about the quality of books published today. I've reached the tipping point on tolerance for this after five years and it's now on the list of reasons I will stop reading your query and say no.
6. The novel isn't finished. Finishing a novel is just the first part of writing it. After you finish, you edit. Then you let it sit for a while. Then you edit again. If you think you're going to be done soon when you've only written 85% of the book, you're wrong. And you're also getting a rejection. Query ONLY when you can send the fully finished and carefully edited novel as soon as I request it.
I can and do overlook typos, getting my name wrong, getting my company's name wrong, leaving out your name and contact info. I don't much like that, and it raises the bar on whether I'm going to ask for more pages, but it does not make me stop reading.