Tess Gerritsen has a great blog entry over at Murderati about how arrogance can kill your chances of getting an agent. The story she tells is not an isolated incident—in fact, it is all too common.
Querying an agent is a lot like going on a job interview. The agent isn't only interested in a writer's talent, but she also wants to know if this is someone she will enjoy working with. No agent wants to work with someone who is pesky, demanding, and think they're God's gift. A humble attitude goes much, much further than overconfidence.