Halona
While I agree with you that a reviewer had better stay away from a disliked genre I'll clearly state that you're plain wrong as far as the other subtopics go.
Given that I'm able to (in my role as a reviewer) present a succint, albeit subjective, opinion about a given work a review will contain at least the following:
1) character depiction
2) narrative
3) adherence to genre
4) good and bad points of the story within the scope of the genre for its supposed demography
Observe that the language used normally isn't part of a review, unless we're talking literary fiction, since mastery of the language ususally is taken for granted. Poor spelling, grammar and sentence structure is basically something new that came along with webpublications. Basically those stories would never pass through the very first filter in traditional publishing.
I did write that a review is subjective. It not only has to be, but it is supposed to be as well. A review is the reviewer's personal opinion about a work, backed up by more or less valid credentials as a reviewer. In the world of reviews some people have a better, or rather more powerful, reputation compared to others. In that sense it's the same as for authors.
For that very reason a reviewer will seldom make it known that they know the author. That kind of relationship undermines the validity of a review, in as much as a review could ever be valid to begin with.
Now, as for opinions about content, making it clear that a story is disturbing due to being "rapey or incest" is perfectly valid. A review is primarily written for other potential readers. It's reader faced, not author faced, and its function therefore is to give a new reader an idea about what is to be expected. If a story carry a set of human values which go contrary to the norm you'd expect to find that pointed out. This would be true no matter if the values presented are worse or better than the norm. Since there's a difference from some kind of standard values the reviewer will implicitly clearly make his/her views of that standard known.
Even if a story makes it abundantly clear that there will be a huge body count concerning innocent people a review that voices an opinion about the depiction of those killings is clearly doing its job. A reviewer stating that a work is a disturbing piece of disgusting crap because it glorifies the raping of recently killed humans is perfectly valid. Reading the review I get an understanding that the reviewer firmly believes there's no excuse for presenting said violations in a rose tinted light. I may or may nor agree with the reviewer (well I personally would agree) and either pick up or not pick up the story based on that review.
The main mistake with the rant I'm currently replying to is that it carries the notion that a reviewer owes the author anything. Nothing could be further from the truth. The reviewer, if owing anyone anything at all, writes for the readers -- the reviewer being one him/herself. As a reviewer I should avoid the idea that an author is some kind of special snowflake and focus on what I believe are the credits or demerits of a story. Nothing else.