1) The overall concept. This includes the plot, the characters, and the overarching themes. Of course, original things are the best and staying away from cookie cutters is essential. It's nearly impossible to have an entirely original idea but it's a good thing if 70% of the book hasn't come from something else out there.
2) The flow. Without any other way to describe it, I'll draw parallel's to a river of chocolate (because who doesn't like chocolate?). The best books have perfectly smooth rivers that float down the riverbed with no bumps or clumps. You feel entirely at home in that river and don't ever feel like getting out. With the addition of some bumps that weren't blended out or bubbles that grow and disrupt the ride down the river, you really want to get out of the river and therefore, stop reading the book. There are plenty of things that jolt someone out of a story. It could be strange wording choices, a completely out-of-left-field choice by a character that doesn't fit their personality or previous actions, or anything in between.
3) The pace. This is pretty connected to the flow. For me to want to continue to read, there has to be something happening and in general, an overarching plot line that continues to drive the plot forward, even when a microplot line ends. If the pace is terribly slow, I don't really want to finish it. If the pace is terribly fast, however, and everything is just thrown at the reader, I may tend to throw up my hands in frustration and that will show in my review. I rarely DNF (did not finish) books but I'll definitely mention pace in my review.
In my reviews, I tend to structure it into bullet points. Generally, these are the characters, the plot, the romance (if there is one and there's something significant that I want to mention), and the writing (this is where I'll mention word choice, flow, and the non-creative aspects of the book).
My rating system goes like this:
Nothing really about this book was appealing to me. It was either incredibly fast or slow paced with frequent bumps in the road and cookie cutter plot, characters, and themes.
There are a lot of things about this book that make me want to put it down but there are a few redeeming qualities. I wouldn't recommend it, but it isn't all bad.
This book was okay. There are a few issues with the characters, perhaps, or the writing, but there was a general feeling of enjoyment. I might recommend this book if it's redeemable qualities are good.
This book is pretty great. I really liked pretty much everything although there were probably a few things that kept me from fully immersing in the story. I would recommend this book.
This book is positively wonderful and I enjoyed everything about it. From the characters to the flow, it was spot on. I would definitely recommend this book.
I also give half stars which, as you can probably guess, mean I thought the book was somewhere in between on of these ratings.