When I review a book I look for a number of things. Character, plot, style and editing. Is the character interesting, do they fulfill their purpose and are they believable. Is the story creative and interesting. How does the story flow and does the story end properly. I look at the mechanics of writing as well. The dialogue, description and if the story is true to its time and place. I look at the overall professional layout and the cover design. I think this is what makes for a good reviewer, professional one or not.