Hmm, you are making me think. Ouch, that's painful.
To be honest, I don't think I ever considered the 'what' that hooks me when I pick up a book to read/buy.
I went through a phase once of reading romance novels and would read whatever by whomever as long as it was a bodice-ripper. But I usually skimmed over the naughty bits because I was more interested in the plot than the heaving bosoms. And they started getting a little boring because they all seemed the same (sincere apologies to all romance writers, this was way back in the late '70s, and I did read 100s of them before I got bored, seriously!!!) Before that it was Dad's old Westerns, never got bored with them, just did the girly romance thing when I got in HS.
SF/Fantasy (vampires/werewolves) doesn't interest me so I never pick one up, therefore I can't speak to that.
What I do love is a good suspense novel, courtroom drama, ghost story, and mystery, old (ala Agatha Cristie) or new (ala John Grisham). As to what the hook is, I am not even sure I know, even though I have been pondering it. For a new-to-me author I will pick up a book, if the back cover looks intriguing I'll check out the first few pages. If that holds my interest, well, the next thing I do is look to see what other books this person has written. This is where it gets weird. I will then pick up the first book by this person and buy that one. If I like it, I will read everything this person put out in order if possible, especially if it's a series or has common characters like those by Elizabeth George, Ann Perry, Richard North Patterson, Elizabeth Peters or Sue Grafton among many many others. I enjoy a book more when I am an 'insider', if you know what I mean.
I can't recall ever starting a book and putting it down because I found issues with it. I love to read, yet I am easily bored, so perhaps I've been lucky. I know there are folks who think the DaVinci Code has issues, but I really liked it, even though there were some over the top bits. Yet the main reason I liked it was because I've been to almost every place in the book, and when I read it I was there, so to speak, because I really had been there. Maybe that's why I didn't see any glaring 'literary' issues because I did enjoy it so much, not that I feel qualified to point them out anyway.
So having reached no profound conclusion, I guess I would have to borrow dgiharris' 'Promise of a good story' as what hooks me (in a genre I like, of course
).
This is unrelated, but there is one other weird thing I do with regard to reading, something I've done forever: A few chapters in, I often read the ending before I finish. I like to see how the author constructs the journey. Yes, I am one of those people who always wanted to be a writer since I picked up my first book but am just getting around to it finally, so perhaps that desire to write was always somewhere in the background simmering. Yet I don't think I ever consciously thought, hey, some day I want to write so I am going to analyze every book I read. (I did it with any book I read, romances included.) Maybe that subconscious 'research' will help me succeed some day!