Agents
I've never been an agent, but I've had three good agents, and I've worked the slush for a couple.
First, I do wish writers would do their homework, and just knowing that the agent handles SF, or horror, or romance is not nearly enough. Read some of the novels the agent has successfully placed with publishers. This is really the only way to know what kind of SF/horror/romance she likes, and what kind of writing/voice/characterization she likes along with the kind.
But most of all, I really wish writers knew how to write an exciting query letter and synopsis. When a writer can't make a 250 word query letter, and/or a 1,000 word synopsis, exciting, there's no reason on earth to think he can make a 100,000 word novel exciting.
Same thing with poor writing in general. When syntax gets screwed up in a short query letter, when sentences read awkwardly, or choppy, in a query letter, only a masochist is going to want to read the novel.
And I really, really, really wish more writers would send along the first three or so pages of the manuscript along with the query. Doing so can pull a bad query letter out of the hottest fire.