Mel Choyce, a WordPress core committer and the designer of Twenty Seventeen, shares thoughts on how good design feedback should be structured. My favorite is probably the last one on the list:
Frame feedback as suggestions, not mandates. “What if you…” and “How about if you tried…” are great ways to present alternate ideas to a designer.
Also don’t miss Mel’s behind the scenes post post on how Twenty Seventeen came to be.