When I was taking my first photo course in high school, the teacher had a rule that every critique had to be balanced.
You had to state whether you liked a photo or not and you had to say why you liked it or didn't like it. Then, even if you hated a photo, you had to point out at least one positive aspect of the picture. Even if you loved a photo, you had to point out at least one thing that you thought would improve it.
That was nearly 30 years ago but I still try to follow that rule whenever I can. I think it's important because it tells people that you are paying attention to the other person's work and giving them the respect they deserve as an artist.