Basic red flags that all publishers despise…
1. Formality—you’d be surprised to hear that simply how the query looks hugely affects the reader’s opinion of whether or not the project is worthy. Besides just basic letter formatting, even in e-mail it should be formatted properly, there’s a tone a writer must strike. Avoid the three C’s: too casual, too colloquial, too cute and anything else that tries too hard to “stand out.” The material itself should be what stands out and no agent wants you be cute about it.
2. Opening lines of the MS—Work like mad on that first paragraph of your manuscript. Sadly, 98% of the queries don’t get read past that. I’m not a fan of dialogue as the opener (though my more commercial fiction colleagues say this isn’t such a no-no). Nevertheless, I tend to delete manuscripts that open with a line of dialogue (esp. one with an exclamation point) and those whose opening line “dumps” exposition. Both of these let me know that you don’t quite have the hang of en media res or of disguising exposition.
3. Clichés in plot summary—These are way more common than you’d think in query pitches: “thought she had it all,” “will stop at nothing,” “must risk everything”—these should be reserved for popcorn flick trailers. When I spot them, I recognize a lazy writer at work and delete. BEWARE.