Good feedback relates back to goals and user needs. Bad feedback is subjective and prescriptive.
For example “There’s way too much going on here and the “Add to Cart” button gets lost.” That’s excellent feedback. Relates to the goal of the page, which is to apparently sell something, and communicates a problem to be solved, which is to get rid of all the junk on the page.
Avoid personal preferences: “I hate green.” There is absolutely nothing I can do with that statement other than feel sorry for you because there are some very nice green things in the world. Like money—which you’re now wasting by giving me bad feedback.
Prescriptive feedback comes along the lines of “Move the buttons over here.” And, of course, everyone’s favorite: “Make the logo bigger!” These may, in fact, be excellent ideas, but if we talked about the problems you’re trying to solve with these prescriptive solutions we might come up with better solutions or possibly uncover a bigger problem in the overall design system.
via “Giving Better Design Feedback” by Mule Design