"If you can't say something nice, don't say anything at all."
I agree with that statement up to a point. While there is no excuse for being rude or obnoxious in a review, we all give our "opinions" on how to perhaps make the writing/story better or at least offer an alternative way of saying something. Most authors on this site understand that the review is to offer suggestions for possible improvement, but there are some who enter the domain who apparently become upset if their work is not praised up one side and down the other which of course is not conducive to learning more about the craft.
I do think an author can learn from basically one sided reviews (what might be construed as negative), but the best reviews cover what is good and what could be improved in the opinion of the reviewer. All we offer is our honest opinions, hopefully. It is up to the author to decide if any suggestions should be incorporated into their work; that is part of the learning process also, but can't be learned if only pats on the back are presented by the reviewer. That "great gob, you're ready for publishing" to me is a disservice to the author even if it is all they are looking for.
Please understand I'm not arguing against what MJ has said, merely expanding the thought process a bit in much the way MJ does in her own excellent reviews. Take care. Vern