The Guide to Literary Agents has a terrific post on agent pet peeves. Quoting numerous agents representing various genres, the post lists everything from cliched phrases ("It was a dark and stormy night" - wow, people actually still write that?) to flowery writing that doesn't move the book forward. Also mentioned: waking from a dream in Chapter One, a run-down list of physical attributes, and characters moving around in the scene, but essentially doing nothing.
Before submitting to your dream agent, check out this article.