I like to think that most books have a natural audience: specific readers whose preferences all but guarantee they will enjoy those books. With that being the case it obviously makes sense for authors and publishers to identify the type of reader who constitutes the natural audience for the books they want to sell. By specifically targetting the appropriate audience no time and money is wasted marketing books to the wrong readers. It obviously wouldn’t make sense to market a book like Altered Carbon to readers who favour Regency romance novels, for example.
If you ask a hundred bookworms to identify the most essential ingredient for a must read book, there’s almost no chance you would get unanimous agreement. All readers have differing sentiments that affect what they want and enjoy most in a work of fiction. For many readers there will be one specific box that absolutely needs to be checked in order for a book to be essential reading. For some, pacing is the priority, for others it’s writing style, for some it’s originality, while for others it might be thought-provoking themes and ideas. I’m not going to attempt to provide a definitive answer because there isn’t one. But I will give you my thoughts on what I consider to be the most important constituent of a good story, the absence of which always detracts from my reading pleasure.