Nobody enjoys reading those flat reviews that just list adjectives, so when I write one, I try to pinpoint the feeling the book gave me and why. Was the pace frantic or contemplative? Did the dialogue sound like real people or like a script? I might mention if a twist felt earned or cheap, not just 'it was shocking.' Sometimes the most useful thing is to note what a book isn't—if someone's looking for a tight mystery, pointing out that 'The Glass Hotel' is more a moody character study saves them frustration. I usually skip the synopsis; the blurb already does that. Instead, I focus on the execution of the premise and who I think would actually click with it.
A small, specific detail can tell you more than a paragraph of vague praise. Saying 'the author uses weather to mirror the protagonist's isolation in chapter four' gives a concrete example of the writing style. I also try to be honest about my biases. If I tend to hate love triangles, I'll say so, so readers can weight my negative take on a romance novel accordingly. The goal isn't to be the definitive judge, but to give enough texture that someone else can decide if it's their kind of book.