I tend to pick books based on their author. If it's by someone I like or who I've heard about, I'm more likely to pick it up. The cover art and title are also important, and I'm more likely to actually purchase a book if it has illustrations I like. I mostly read fantasy or classics and sometimes horror, and I actually used to pick books based on their length, because I liked long fantasy novels. (The Wheel of Time kinda burned me out on that, so that's not really a criterion anymore.)
Also, if a book has any kind on genderbending theme, I'm probably 5x more likely to pick it up