A good mystery is easy to spot. Great writing, entertaining characters, and a solid case with clues, detecting, and a satisfying resolution suck the reader in and carry them to the last page.
But what makes a bad mystery? Turns out there are several ways to ruin a perfectly good book, besides just a poor writing style.
Weak plot can easily kill a book. If I'm a hundred pages in and still not sure what the story is about, there's a problem. On more than one occasion, I've found myself flipping to the back cover just to remind myself what the overall arc of a story was. I've done this enough times that I started to doubt my own reading comprehension skills. But a quick trip to Amazon to read others' reviews confirms that the author simply didn't plot their book out well.
Another sign of a bad mystery is when the side plot dominates the story. I've just begun a mystery where the first twenty pages are devoted to a new bride meeting her husband's family. While twenty pages isn't a lot, it's obvious that this subplot is going to play a major role in the book, more than likely overshadowing the main plot.
And don't get me started on books where I spend the entire time shaking my head at how preposterous the plot is. It's fine if the story occurs on Mars or the detective is a blind Las Vegas showgirl who used to be a nun, providing the author can lay the groundwork and make the plot believable. When the author can't be bothered to make an odd situation seem like a reality, I'm too distracted to care about the mystery.
And what of those pesky murder mysteries that contain no clues until the end? I find myself in the last chapter without any indication as to who the killer might be, and suddenly, the detective (usually an amateur one) finds a tell-tale photograph or a scrap of clothing and says, "Aha, I know who did it!" That's not fair! The reader needs hints along the way. I'm not reading this book for my health. I'm trying to solve the puzzle. Or worse yet, the killer's motive is revealed and it's a minor infraction. The woman killed three people to hide a secret that involved her great, great grandmother? Not likely. It's so frustrating to read a fantastic book that closes with a whimper instead of a bang.
Boy, in looking at the paragraphs above, it's a wonder anyone bothers to write a mystery at all with so many ways to screw it up. And while I whine about other books I've read, who's to say mine doesn't suffer from the same problems? Just because the plot makes sense to me doesn't mean it will to a reader. A reader can't see inside my head for an explanation of those confusing spots. As for my side plots, do they dominate the story? Or have I spent too little time on them in fear that I'll bore the reader? I have no idea. But I'll keep reading those great mysteries for inspiration and guidance. And those bad mysteries too, even if I do pick them apart. Because even a bad mystery can be entertaining.