I was selected to represent my high school as a candidate for computer science at a state-level competition. The teacher in charge of selection made it clear that he very much didn't want to pick me -- there just weren't any better options. He explained that my portfolio was all based on user skills. Nothing showed a deeper understanding of the underlying principles of computing. I didn't incorporate any of this feedback and was thoroughly humbled later on. It was a well-deserved loss.
Twenty plus years later and I've still never had more useful interview-related feedback and I'm still grateful he was willing to share that criticism. Now, having been on the flip side of the process quite a bit, I especially appreciate how hard it is to provide meaningful "negative" feedback.