One of the more commonly used methods to figure out if an unknown book is for you is the first page test. Or first chapter. Or, occasionally, the first sentence test. It's this little bit of security before buying a book - can I even read a little of it without puking? It works pretty well, for the most part. I'm a fan. But sometimes... sometimes it goes wrong.
I'm reading a book now that passed the preliminary test. And it was on sale. An unheard-of publisher (possibly independent or self-published?), a book I'd never heard of before, a strong opening chapter, and an interesting synopsis... I bought it and immediately dove in. The thing is, for the first two or three chapters, I was really into it. The topic is something very relevant to my life right now, but from a different angle - at once something I know nothing about and something that consumes my day-to-day life. In other words, a seemingly perfect combination for a novel that is clearly very much based in fact.
But the book, like so many others, began to go downhill. Whether or not it will ultimately disappoint, I don't know - I have another quarter left to read out of this fairly small book. Another evening or two, I expect to be done. But now, approaching the end without approaching the feeling of an ending (there is no sense in the writing that a climax is to come), I find myself angry at the author. Because when you start out so strong, it's infinitely more disappointing to see the inevitable downfall.
Sometimes books go bad because of their characters. The characters are flat and don't evolve throughout the book. Sometimes it's the plotting that kills a book, just going nowhere. And sometimes, a few sad, painful times, it's the writing. And not all the writing, just small parts. Take the above example - the writing is good when describing situations and actions. Characters are believable, easy to imagine. Two problems:
1. The characters all, with a single exception, speak in the same overly fancy, unrealistic voice. They speak unnaturally, using stiff phrases and drama to emphasize their emotions. Ultimately hard to swallow. Only one character (a friend of the narrator) sounds different, most likely because he fits a common stereotype - humorous, casually charming, loyal, kind. He sounds like one would expect him to sound. The narrator, and every subsequent character, sound like they're having too much fun using vocab words.
2. Plot points and scenes that serve no purpose in furthering the story. Intentionally explicit scenes are clearly meant to spice up the story, but the impression is one of awkwardness and sensitive subjects are completely ignored for in-your-face sexual style story-telling.
Two examples for one specific book, but every book that finds itself going downhill has different frustrating points. Most books have it, but sometimes it really jumps out - the small things that turn a book from great to simply mediocre. On rare occasions, you can set aside the downsides, chalk them up to style or say they pale in comparison to the glaring positives, but sometimes there's just a downfall and every time I find myself disappointed like new.