And another part of this seems to be that people don't want to be the first one to say anything.
That same review board I linked above, if you go to the next page, the first reviewer states that they had issues with the story as well, but also indicates because all the other reviews were positive, they didn't want to say anything. That tells me that a great many more people would review if there was some type of "this is good, but it could be better" tips were left (but not with that wording - that's presumptuous, and shouldn't be used).
And of course the review right behind it is fanpoodling and doesn't actually have anything to say about the story itself, just complaining that the author is getting the concrit that was requested.