1) Clarity- Make sure the editor understands your description of the plot, and the conflict your protagonist faces.
2) Brevity- Keep your query word count under 170 words, or at the very least under 200. Use as few words as possible to entice the editor and make them WANT you.
3) Logic- Subset of clarity. Make sure your described plot makes sense. Example of illogic: "Joy is a twenty-three year old millionaire working as a prostitute as a penance to ease her guilt over the death of her ex-boyfriend." (This is verbatim from the QuerShark blog.)
4) Voice- Give an inkling of the tone your novel is in. If it's a thriller set during the Cambodian massacres, write a fast-pasted query. If you're writing a satire involving cucumber gondolas and evil overlords wanting all the world's cheese, incorporate a few biting phrases. Stephanie Meyer and J.K. Rowling are famous because they have writing styles that flow into their reader's mind; add that verve to your novel!
5) Consistency- My additional rule. If you write the query in first-person (that is, the protagonist's voice), make sure your novel is in first person. If the editor likes your query, have the novel ready for them. Can also be labelled at EXPECTATIONS.
Need to go now to slice some oranges. Tootles!