That doesn't mean all of them, of course.
I don't like the ones that are too silly. I can't stand the amateur who should stay out of the way and let the police investigate. I don't like the best friend who gets the protagonist into trouble with her antics.
I want some sense of reality in the world an author creates.
That's why I like MACDEATH. The main character is real, funny but not silly.
That's why I like PLAN X. The hero has things to deal with. The action moves quickly. Nobody gets waterboarded.
I don't like mysteries where no one is likable. I recently finished one where everyone in the book screamed insults at each other through the whole thing, and I wondered why I bothered. Yes, I wanted to know who the real killer was, but it wasn't really worth it in the end. Time spent with mean, nasty people never seems like a reward. That's where NINE DAYS TO EVIL is good. You like the main character immediately, and you want her to get her life straightened out. I need someone to cheer for,
or a book is disappointing.
And if you know me, you know I love history. That's why I like (and wrote) SHAKESPEARE'S BLOOD. Like the other books, it's set in modern times, but there's a connection to one of my favorite plays, Macbeth. It doesn't hurt to learn a little bit from reading a novel, even if it's alternative history.