The point of that tag though, N/C or the more recent Rapefic, is as a warning. Rape is a nasty thing. It is dark, and it is disgusting. Leaving the author a negative review because you had a normal reaction to rape? Well, at least the author knows they wrote the rape scene out well enough that it's believable.
I honestly don't feel that there is a possibility of forgetting that non-consensual sex is rape, and I've read/skimmed stories that have tried to romanticize it; however, being as this thread is about reviews and not how one perceives the violent degradation that is forced intercourse, I'll not go into details.
Suffice to say, that, differences of opinion on that aside, I don't think leaving a negative review when it is clear the author intends the piece to be dark (by labeling it N/C or Rapefic) is necessary. I've been told that JayDee writes wonderfully, that the stories are always plotted well and the scenes are thoroughly riveting. However, because of the subject matter chosen - and I take advantage of the warning tags that are provided to authors for use in summaries - I refrain from looking at JayDee's work except for sorting purposes for cleanup.
As I said above, the most that you're accomplishing is telling the author that they've accomplished recounting the very negative aspect about which they first warned you. Having said that, negative reviews can fit into the categories described above as well.
I didn't like this clearly gives the author no other feedback and is not helpful to them as a writer.
I read this because I thought you would romanticize the point where Draco rapes Hermione, but I see now that isn't the case tells the author what you didn't like about it (and a bit about you as a reader as well).
Both of them are short, but at least the second one, though negative, tells the author what part of the story had a negative impact.
And I will, very sincerely, tell you that I try to leave at least some "criticism" in my reviews - I think this may be where people refrain from doing this. Criticism isn't just "you had this wrong" (type 2 in the first post); it's also when you tell an author that they wrote this part believably, when you tell an author that the point at which character A interacted with character B was done well. Positive criticism is still a critique of the work, but when people see that word, all they seem to think is of its negative connotations.