1. The font is tiny and single-spaced and the margins are miniscule. That lacks professionalism and tells me that you expect me to wade through something that is not quite legible.
2. There are mechanical errors from the get-go.
3. It’s boring. When something is boring, it’s usually because it lacks pacing. Give me something, anything, to worry about.
4. It doesn’t fit the genre it’s supposed to be in.
5. It’s too slow. Which is boring.
6. The characters are dull. Boring.
7. It doesn’t appear to be about anything in particular. Where’s the story? When you don’t introduce a story promptly, it gets boring,
8. It lacks voice, which makes it boring.
9. The cover letter is hostile or tries to crack inappropriate jokes or is disrespectful. One once called me “an arrogant gatekeeper”.
10. It doesn’t know what it is. Where would it fit on the shelf?
The word I used most here is “boring”. That is the number one reason something gets declined. Hook me right away and keep me there.