I am all three, so I’m going to hit the topic hard as I’ve already answered the question as to why rating books * 5 Stars * is important.
While what constitutes positive, and constructive, or upbeat is subjective, the importance of encouraging non-readers to want to read, plain and simple, means talking up books that are appealing.
It’s the same as if trying to sell a home, or getting a friend to attend a party...or come to your event.
You wouldn’t tell this person, “hey, this house sucks. It has old plumbing, the roof leaks, there’re few closets, have itsy-bitsy rooms, and it collects dust,” believing this is motivation to encourage a buyer to buy; even if it might be motivation to the owner (the author) to take better care of the next home they try to sell.
Same goes for trying to get someone to hang out or come to an event. It would be like cutting off your nose to spite your face to point out all the reasons why “anyone” would likely not want to hang out or come out believing things like “hey look, at least I kept it real and honest.”
My point is, it is important to talk UP books we enjoy if the idea is to encourage reading. Turning readers away from *a* book could be, turning readers away from reading *your* book, in addition to encouraging readers to continue with activities that appear more worthwhile. Hence, save the honesty to send in a personal email... that is... and only if... your desire is to PROMOTE READING!
Reading Between the Lines... Point out what's working in the story and review that. If nothing is working, find a book that is working and talk that one UP!