1 - Spoilers. I admit it, I take spoilers personally (probably a weakness on my part). I consider myself a potential reader for many novels whose reviews I read, and if there's something there that destroys rather than enhances my future reading experience, I get upset. In cases when I'm unlikely to read a novel myself, I put myself in the shoes of potential readers and guess what their reaction would be.
An example - I once received a review of a 20th century war novel that said the brilliant, heroic protagonist would die in a fiery plane crash on the very last page. Sometimes these things are foreshadowed in the novel's introduction, but this one wasn't. Most spoilers are not so blatantly obvious, but it's always better to blur the details. In this case, it would've been better to say that the dramatic finale would shock readers, or something similar. The reviews in Kirkus occasionally give away major plot developments, so I'm wary about reading them if I think I might read the book later on.
With historical fiction, well-known historical facts aren't spoilers... people generally know what happened to Anne Boleyn, the Donner Party, etc. With lesser-known or fictional characters, I'm much more careful.
(Sometimes, I admit, I do sneak peeks at a novel's ending, either because I'm curious or because I'm bored and want a reason to keep reading. But that's my decision.)
2 - Overly pedantic reviews. One reader's historically sensitive review (see previous post, point #3) is another reader's pedantry; historical inaccuracies that bother one person may not bother another. It depends on the reader's own historical background, how forgiving s/he is toward errors, and how big the errors are. As I've said, I appreciate reviews that point out major historical blunders, ones big enough to draw a reader out of the story. (Insert disclaimers here about the many different ways history can be interpreted, the presence of authors' notes that explain how an author diverged from history, etc., etc.)
But with regard to errors in particular, let's not go too far, especially in reviews restricted by length. I don't expect reviewers to read with a specialized dictionary in hand, in the hopes of catching the author in a mistake. Pomposity in reviews often has me siding with the poor beleaguered author.
3 - Too much personal information, either about the reviewer or the author. Again, most reviews have limited space. It's helpful for readers to know, for example, that Innocent Traitor is Alison Weir's first work of fiction after writing many historical biographies. It's not as helpful to know that Joe Author retired from his longtime insurance job and discovered a 2nd career as a novelist while living on his houseboat in the Florida Keys.
4 - Reviewers with an axe to grind, or other obvious mismatches between reviewers and books. The New York Times (as well as other broadsheets) knows that the former often results in entertaining reviews. And so it does. But as a reviews editor, I try to avoid these situations - when I know about them, that is. I don't want to send feminist biblical reimaginings to reviewers who are religious purists, because I want the book to get a fair reading. The Booklist Online blog - take a look at the June 8th entry - has a really interesting piece on the art of matching books to reviewers.
5. - Reviews that demonstrate that the reviewer didn't "get" the book. I'm continually surprised, for instance, at the number of reviews from Publishers Weekly that review mainstream historical novels as if they were historical romances. Examples (which lead to the Amazon pages with the PW reviews): James C. Martin's Push Not the River and Anne Easter Smith's A Rose for the Crown. Both have romantic elements but are not genre romances. The overall review of the latter wasn't bad, but no novel about Richard III's relationship with his mistress will have a happily-ever-after ending; is this any big shock? The PNTR review got several other facts wrong, including the author's gender (noted by bracketed text in the Amazon version).
OK, whining mode off. Questions: what bugs you in reviews of historical fiction? Would the factors I listed above be turnoffs for you as well?