In the very early days of Pixar, John, Andrew, Pete, Lee, and Joe made a promise to one another. No matter what happened, they would always tell each other the truth. They did this because they recognized how important and rare candid feedback is and how, without it, our films would suffer. Then and now, the term we use to describe this kind of constructive criticism is "good notes."
A good note says what is wrong, what is missing, what isn't clear, what makes no sense. A good note is offered in a timely moment, not too late to fix the problem. A good note doesn't make demands; it doesn't even have to include a proposed fix. But if it does, that fix is offered only to illustrate a potential solution, not to prescribe an answer. Most of all, though, a good note is specific. "I'm writhing with boredom," is not a good note.Catmull quotes Andrew Stanton at length explaining the difference between criticism, and constructive criticism, ending with: It's more of a challenge. "Isn't this what you want? I want that too!" 
I think this bit is the key: good criticism focuses on the common goal: a great product. It inspires, rather than creating defensiveness.
I read Reviewboard feedback in a sort of random way, and see a lot of "good note" behavior. But that timely part is sometimes missing. We have some Reviewboard requests languishing, along with patches in bug reports. Fortunately, the Gardening project has sprung up to improve this part of the community. Help out if you have time! https://mail.kde.org/mailman/listinfo/kde-gardening and https://community.kde.org/Gardening.