Here's your answer, its the email i sent to a writer friend this morning:
Subject: You Damn Theif
Body: You owe me five hours sleep. "***** ******" was awesome.
We want a book to invade, attack, lay siege and occupy our attention for hours, it needs to overrun the other stuff we need to be doing. If there are flaws to the book, they need to be stuff that doesn't compete with the entropic advancement of the story.
The story does also have to be marketable. Given the pushback of readers and the ennui of agents and editors over vampires, particularly the sparkly, PETA-Greenpeace-Emo variety, even if you turn in a captivating novel who's only flaw is that the barely-legal-centuries-undead male lead is the type who likes chick flicks, reads poetry and sleeps with a teddy bear, you're probably not selling the book.
And while simply holding the attention of the average reader can be difficult, remember that no agent or editor is looking for a reason to publish your book. They are looking for all the reasons they shouldn't. Also, no agent ever lacks for things to read. Be it slush, client proposals, new books from clients, industry news, or the map that might help them relocate their scattered wits; it's far easier for an agent to put down a book because of how much 'free' stuff they have to read.