1. LANGUAGE IS AWKWARD OR GETS IN THE WAY OF MEANING
From a query I just received today:
An event is described and then: "The ensuing tale follows the events and circumstances that lead to that horrible culmination." I find this confusing, as if it's a circular story somehow... except I don't think that was the writer's intention.

The purpose of your hook is to give a clear idea of what your story is about. You can forego fancy language and unwieldy sentences and just state it clearly. When I see a writer struggling with how to express himself in a hook, it doesn't give me confidence he knows how to use words comfortably in a novel either.

2. IT DOESN'T BOLDLY GO WHERE OTHER WRITERS HAVEN'T GONE BEFORE
I see a lot of plots with familiar rings to them. I want writers to dream up a complex situation I haven't seen a lot of times already. Something new, something fresh. A new slant, an intriguing character, a cool setting, a story I'm dying to read. Dare to dream up a truly different idea.

3. IT BOLDLY GOES WAY TOO FAR
Okay, but not aliens kidnapping Santa for the devil. I don't mean outrageous, I mean creative. Something still human enough that readers can relate to it. Something with universal themes but told in a different way.

4. THINLY MASKED POLITICAL RANT OR MORALITY LESSON OR PERSONAL FANTASY OR JOURNAL
Yes, writing is cathartic. Political satires are tough though. So are illness memoirs. Personal fantasy of ditching your job and family responsibilities to go on a road trip and meet a young sexy new love... also done a lot and not particularly marketable. Sure, it's a common theme... but without a new twist it's not going to stand out from the rest.

5. IT COMES DOWN TO AN AGONIZING CHOICE
I've seen whole novels work as a buildup to the ending of a huge decision the protagonist must make... and choose wisely--make a selfish scared choice, or make the riskier self-sacrificial choice. I'm more interested in seeing what the protagonist is up against, and then have the question: How will she keep what she dearly wants but still rescue the man she owes her life to? So the question is how can she get everything she wants with the smallest casualty. I like how questions better. I'm not so interested in a choice that's A, B, or C. I want to see the protagonist choose D. None of the above and make her own way. If your book isn't so straightforward, don't let your hook suggest it's that simple either.

6. CLICHES
Visions and prophetic dreams, a quest to find a person or object, a portal into a new world, a girl who goes on disasterous dates until finding Mr. Right, etc. Read a lot. Know what's been done already so that you can do something new, different, better.

7. UNLIKEABLE CHARACTER
Get my sympathy or admiration, and I'm hooked.

EXTRA BONUS ADVICE:

ANALYZE THE GOOD STUFF
Great storytelling comes in all forms, and you should be paying attention to it. Read a lot. Watch acclaimed dramas. Analyze. I'm watching the first season of SIX FEET UNDER on DVD, and this does everything right: Characters are sympathetic because they've just had a loss. They're a family so they have to pull together even though they're all different from each other and each have their secrets. They have an interesting job--funeral home. Universal resonating themes of family connection and grief. Issues of "coming out" to your family, struggling in school, trying to save a family business that the big corporations are trying to take over, coming to terms with an affair when the deceased isn't around anymore to forgive you, starting over after a spouse dies, losing a child to gang violence, not knowing what to say to those suffering a loss, etc. A lot can be learned from other excellent storytellers. (TV and movies can get away with more generic themes than books, so don't replace all your reading time with other media.)

EXERCISE YOUR CREATIVE MUSCLES
Make up stories all the time. Don't allow yourself to be so sucked in to one long-term project that you stop inventing new stories. Yes, you have to do the hard work of writing each word that goes into the book and then edit, polish, edit some more. But meanwhile, you need to keep creating. Use your moments driving to work or your lunch break to watch people and imagine their lives. Read the paper and imagine taking an article as inspiration for a novel. Combine a few. Keep a notebook of quirky characters. Invent a setting and imagine who would live there. Next time you disagree with someone, imagine what that person's thinking and see if you can invent the story from their justifications, their point of view. Get outside your own head this way. If you come up with the spark of something truly special, you'll know it right away. It will stand out from all your other ideas... and that means it will stand out from all the other hooks too.

PAY ATTENTION TO WHAT MOVES YOU
What makes you mad, grieves you, thrills you, hurts you? What makes you feel betrayed? What's something you've sworn you'd never ever do? Imagine someone who does that and force yourself to see it from their point of view. Making a character go against their "type" is a powerful thing. Weave these emotions into your stories.

PICTURE SOMETHING ACTIVE
Action works. Unusual action always works. Someone films on the street around here, and there's always a curious passerby who asks what movie's filming. If there's a wreck, drivers rubberneck. Street performers juggle fire or do a back flip over a trash can. Imagine someone in a scenario that will make readers stop to watch. We're all starved to fill our eyes with extraordinary things--dream us up something extraordinary. Imagine a crazy embarassing scenario--for some reason a woman wearing tennis shoes and a formal dress with purple paint on it is stuck at the top of a rafter. If someone started a story that way, I'd want to know how she got up there, wouldn't you?

Imagine. Create. Write.

As for me, I'm getting back to submissions. (Several of my clients have finished their edits and their work is ready to get out there.) And I have to tell you, writing a pitch for a great book isn't nearly as tough as writing a pitch for a book with no stand-out elements.