You know what a phrase I really don't like is? "You can't judge a book by its cover."
When it comes to actual people, to whom the metaphor is often applied, that is true. Books, though. Until you open them, the cover is all you have to go by! Is it a hardcover or a paperback? Is it a cardboard hardcover, or leather bound? Is it a nice fat trade paperback, or one of those weird, tall floppy ones? Does the hardcover have a dust jacket on it? Is the cover glossy or textured? Are there people on the cover picture, or is it a landscape? If there's a dog, is it the actual breed that the story describes (this you can only find out later. And, mostly, it isn't.) Does the book smell bad?
The cover is what makes you pick the book up. The title is what keeps it in your hands. After that, though, methods vary. Some people read the blurb on the inside cover (hardback) or on the back cover (paperback). I never do; those blurbs tend to give away key plot points that I would prefer to actually discover in the course of reading. Some people read reviews on either the cover or inside. This is okay, I guess, but sometimes, reviews are bought. It's a sad fact.
No, my third step to picking a book is to read the first line. If that grabs me well enough, then I go on through the first paragraph and first few pages, until I'm either sucked in, or know that I'm disinterested. I can't say that any one specific thing will make me put a book down every time, but there are some basic ones.
For instance: I don't mind reading a book in first person. I know some people do. But there's always the awkward application of the narrator's name. In Daphne DuMaurier's Rebecca, you never learn the narrator's name, and it doesn't matter. In other books, the author interferes, breaking the fourth wall and having the character announce to the reader "Hi, my name is so-and-so!" I feel, unless that's what your book is about, you do not break the fourth wall.
Perhaps most importantly, you are not honor-bound to finish every book you start. Another time, I will regale you with the tale of the worst book I have ever read; suffice to say, I swallowed that bitter pill, and have released myself from the duty of finishing a book if I realize halfway through that it's ridiculous, or I'm not enjoying it. That's an interesting freedom, and thankfully, it doesn't come up often, but boy howdy.
Of course, in that particular instance, that book's cover should have told me that it was not a book I was looking for. But I didn't listen.