I like to get a very general idea what the book is about, and get a feel for what impact it had on the reader. Was it funny? Did it make you cry? What issues did it make you think about? That sort of thing.
Book reviews should tell the reader enough about the book to let them know if it's the kind of book they might enjoy or get something out of or whether they ought to avoid it. They shouldn't give stars or best stamps because reading is a very personal thing. Whether we love or hate a book shouldn't be in the review - unless you state it's a personal response.
So, a book review should give basic plot, something about style, structure, characterisation, maybe a quotation (not too lengthy) to give an idea of the narrative and should give a summation of whether it successfully achieves what it sets out to do.
A book is what it is, that's what ought to be judged.
A book review is not a synopsis. I've read reviews that precluded my need to read the book because the reviewer told all the juicy highlights leaving nothing to be discovered. A review should be composed of some remarks about the authors style, character development, and the emotional/intellectual impact of the book on the reviewer.
I think they should be (not too short), but to the point. I also love it to be less professional. I want to feel like I have just been told by one of my friends that this book rocked. (Spelling and grammer are still important!)
A good book review give a glimpse of the story without giving away too much. I like when the reviewer tells me if they liked it. If the reviewer is excited about the book and it sounds like a plot I would like I am more likely to read it.
I would include a quote or two from the book or an interview with the author to give the reader a small taste of the work. I think detail like that helps a book review but in order for it to be a truly useful review the writer needs to have an informed opinion to share with readers. Did they like the book, why or why not should be the central question the review answers.
A good review does not give away the book, but instead lets those reading the review how you, as the reader felt over all. I look strongly into character development and like to have a good sense of who they are not only physically so I can get a clear picture of them in my head - but also who they are as people. I want to know if a character struggles with a choice or has fear, or strong beliefs. Is the character wimpy? Then I want to sense and feel their weakness. Is the character strong and determined? Then I want to battle right along side them - or at least feel as though I have.
In a review - I look for a good feel of what sort of style the author writes in. Does it reminder you of another author? Then I want to know. Is the author unique in his/her writing style? That's a huge plus as too often we get trapped in a cookie cutter style of read....
I agree with some of the other comments here.... share how the book made you feel without giving the book away. Dont make it overly long and do not give too much away.