1. Grammar mistakes I haven't seen since 4th grade.
How you will avoid this: If you know you're weak in the fundamentals of sentence construction, take a class. Proper grammar is essential. You don't have to use it, but you have to know it. And if you use bad grammar it should be on porpoise. Like for a laugh. Or to make a point. Or to convey a character's voice.
2. The premise of the novel was so distasteful I wouldn't even want to know the writer, let alone work with him/her.
How you will avoid this: Hard to say. You might run the idea past some readers and watch their reaction. Otherwise resign yourself to Lysol-scented replies.
3. You flat out told me you disregarded the query guidelines.
The problem isn't that you did so; I get queries every day that do. It's that you added "I don't have time to do this kind of folderol" which tells me you're impatient and convinced you're right about everything (including stuff you don't know anything about.) This bodes ill for your career path in a new industry. Make no mistake about it: being published is a job. Being a writer might be about art, but once you want to be published, it's a business. Would you hire a person who flat out told you the job application was beneath them?
How you will avoid this: if you don't follow the directions, keep it to yourself. If you think agents are witless, mercurial, and savage beasts, put here on this earth solely to torment writers, well you're right of course, but keep it out of your query. In other words, pretend to be a polite person I'd want to work with.
4. Misused words/homonyms
I literally stop reading your work if I see more than three of these in the pages you include with your query. I've ranted about this at length in other blog posts. No matter how good your story is, I can't read it if I'm frequently drawn out of the narrative by thinking "wait, she means alley here, not ally."
How you will avoid this: Have a beta reader who could double as a grammar velociraptor. Pay someone if you have to. Spell czech Will Not Help You!
5. Pages are not compelling
Your pages need to entice me to read more. It doesn't have to be with some sort of wildly dramatic event, although that usually works pretty well. This is more like you create a world I want to see more of, a world I want to explore with you. Dennis Lehane is a master of this.
How you will avoid this: study a novel that you love. When I say study, I mean close study, as in typing it out in its entirety or reading it aloud yourself. Watch for how the writer entices you to read more. Then do that. All great artists learn from those who came before. What you're doing is the equivalent of art students sitting in museums re-creating the paintings of the great masters.
One of my favorite pieces of writing advice is from Elmore Leonard: Take out everything that sounds like writing. Here's an example "his gaze wandered to the television."
6. Your query was brilliant; your pages not so much.
A good query entices me to read the pages you include. Those pages have to be as good or BETTER than the query. I don't request a manuscript if the pages don't entice me, no matter how good the query is.
How you will avoid this part one: Have someone read your pages. If they ask to read more, you're on the right track. If they don't, you know you need to do some work.
How you will avoid this part two: don't ever send a prologue if it's markedly different in tone and voice from the book you describe in your query. It will just confuse me. Send it ONLY with a full manuscript request.