In this case, the author (who shall remain nameless, though I will say it was not someone I know from my own publishing house nor anyone who is very well known) wrote a very interesting and compelling story with characters who were very well-drawn and with whom I found it easy to feel a connection. So why didn't it rate five stars?
Most people equate the words "bad writing" with "terrible story" but that's not necessarily how it works. We all know people who have a great story to tell but bad story-telling skills. They wander from the main story to tell interesting side tidbits (well, interesting to the storyteller) or stop the story to explain the intricacies of the relationships between the persons in the story or they're in such a hurry to get to "the good part" of the story that they gloss over or confuse the details that make the story so compelling. A bad storyteller can take the best story and make it dull, boring, confusing, or unintentionally funny or sad and frustrate their audience. Likewise, there are many ways a writer can make a reader want to put a book down and go watch TV instead.
"Must... save... readers... from... TV...."
Also, it became apparent that the author had a tendency to switch back and forth between POVs (points of view)... sometimes in the same paragraph. The transition is a bit jarring to suddenly jump from one person's head to another's without warning. POV shifts should not be confusing; readers should know at all times whose head they're in. Some writers are masters at omniscient point-of-view and can manage a large cast of characters without losing the reader along the way. Many, like me, can't do this. It's best to stick to one or two POVs in a novel than try to force the reader to get to know everything about every character from the inside out.
The author also had a habit of switching from past tense to present tense, especially when talking about real-life locations he uses to add local color to his fiction. It's perfectly fine to mention a restaurant that actually exists in the real-life city you're using for a setting--it certainly sets the scene well, especially if readers are familiar with the location you're talking about. What is not okay is to suddenly switch to present tense to add a "public service announcement" in the middle of a scene:
"Five minutes later, she stepped into his office with an appointment to meet him at the restaurant for lunch at 12:15 today.
The restaurant is one of those places that succeeds by providing good food and good service and doing it all the time. The manager has started several other restaurants in town using the same strategy and has never needed gimmicks in spite of the high overall failure rate in the city's food service industry."
Nice to know (even if the restaurant alluded to in the book has now gone out of business) but it doesn't add anything to the story. The characters could have been meeting at McDonald's and it wouldn't have made a difference.
These are only a few things that made my experience of reading this author's work less than satisfying. I really was pulled in by the characters and the story, but unless the author brushes up on his writing skills, I doubt I'll spend my hard-earned money on another book of his. But as a whole, I still found the experience rewarding. After all, I learned what the author did right and what the author did wrong and I can always use that knowledge to improve my own abilities.