Here are the general things that I have found that kill my interest in a hook:
1. Everything is too generic. The story and characters just seem to be a composite of existing published books in the genre, with nothing new.
2. No emotional connection with the characters. Or worse, the reader quickly comes to hate the characters. We have to see these characters as actual people, and they have to be in situations that one way or another make us identify with them in some way. They can't be too godlike, or nothing but flaws.
3. The plot/setting/premise seems flawed or unrealistic. Generally speaking, when someone reads your hook, what you tell them is all they know. So when key information is omitted, or your description is such that it seems like there is a simple and obvious solution (even though the obvious solution might well be disqualified in the book itself), that seems like a logic error to the reader, and that's pretty fatal.
4. Overly wordy. Too many hooks just aren't economical with word choice, and so the rhythm and clarity of the writing both take a huge hit. Even writers who are excellent in longer form will freeze up and have all sorts of awkward wording in this ultra-short format.
5. Voice and style get lost. In too many hooks, perhaps even including my own, the writer is so focused on efficiently conveying information that their own voice and style get lost. It can make for a rather boring or monotonous read if care is not taken.
6. The presentation is too complex. Complex ideas are great in books, and one of the beautiful things about books is that they are long enough to express these ideas while still telling an engaging story. Unfortunately, hooks are far too short to do this, and from a standpoint of sales it seems like the engaging story part wins out. Hinting is complexity is good, and striking new ideas are great, but trying to explain too many plot twists or too much of your complex worldbuilding can really bog down a hook in a bad way. Remember: you're obviously going to have to leave some parts of your book out of your hook--otherwise there wouldn't be much need for a book-length version, would there?
7. The material is too simple. There were a few hooks posted that were just way too short. They were on the order to 50-100 words, and just did a little bit of description of the world and the overarching problem. Some didn't even mention any specific characters, which is obviously a problem. Even though you can't (and shouldn't) outline every part of your story in your hook, there needs to be enough there to properly hint at all the great things that await those who read the entire thing.
8. Misspellings. This one seems pretty inexcusable to me. This is a short enough piece of text that it should be highly polished. If you've got one or two misspellings in your hook, it's a red flag that your ms might be littered with them. It seems like a lot of people don't know the difference between "lose" and "loose," which is really a pet peeve of mine. Almost as much as "their" and "there" and "they're," and "lead" versus "led."
9. Rhetorical Questions? Have you ever had your eyebrows plucked by a superhero waitress? It's really irritating when a question is asked that has an obviously negative answer. Or, if the answer is obviously positive, it still begs a negative answer from the irritated reader (Have you ever wished you were fabulously rich?). Maybe this doesn't bug some people, but I know that agent Nathan Bransford has commented on this several times before.
10. Self Praise. This just doesn't work. Discussing how brilliant you think your book is, or all the wonderful lessons that it has to teach humanity, just doesn't come across. You can't claim that your own book is smart or funny and have a reader believe you--you have to show them. Really, this is back to that "show, don't tell" mantra.
So, that's my ten things (in no particular order). Now, which ones have I fallen prey to? In THE GUARDIAN's hook I had a problem with #1, having things sound too generic. In ALDEN RIDGE's current hook, I'm perhaps a little bit too distant from the characters, thus indicating #2. In the past, I've had a lot of trouble with #4, being overly wordy, but I hope I'm past that now. Item #6 (too complex) has been something that I've intentionally been avoiding, but that often makes me fall prey to #7 (too simple). It's a fine line, and I have yet to find the perfect balance there. At least I don't do any of the others (that I know of).
Hook writing is a tricky business, and in many ways requires different skills than actually writing a good book does. But it's one of those necessary evils, so we all have to get really good at it. That's why I started so early with my hook for ALDEN RIDGE--it gives me months and months to rethink it and revise. There's nothing worse than spending a year or more to write your book, then a month or two to polish it and make revisions based on beta reader feedback, and then sending out queries with a crappy hook that you wrote in a few hours or days. That's what I did with THE GUARDIAN, and that was a mistake. Please don't do the same!