what is the minimum one should reasonably be able to expect from a reviewer?
1: The reviewer will have read the book that they are reviewing and will review that book, not an entirely different version that they may have dreamed up after a visit from Jacob Marley .
1a: Reviewing a book in terms of how it isn't the book you were hoping it would be but rather the book that the author actually wanted to write is probably a bad idea. Examples might be a recent book where I hoped I was going to get a book about a rock band but got something else instead. The book was supposed to be about something else and my expectations were inaccurate.
2: It's probably best if the reviewer has no strong opinions about the author (or anyone else involved in the production of the book) one way or another. At the least, to be aware of them so that you can compensate.
There's a bit in Machiavelli's Discourses on Livy about not allowing flaws to prevent someone's virtues from being rewarded and not allowing virtures to deter one from punishing transgressions that might apply here. Actually, I am not totally sure he said the first half. It's been a long time since I read Discourses.
3: There's nothing wrong with having preferences but those preferences do not constitute natural law.