# 1 on the list: Typos.
"...especially in the query letter, and mechanical errors in the first page of the synopsis and chapters...Typos jump right out and attack the eyes of editors and agents, and you don't want to cause that kind of anguish."If you have a moment, pop over to the Seekerville blog and read her full post. It's great insight about the things that will cause an editor or agent to reject your writing.
And no matter what you write, always, always take the time to carefully proofread your work before sending it out to the world. Your readers will thank you for it.