I generally see the best features of anyone's work and if required (asked) to comment, I point out the positives. If someone genuinely wants to have the flaws noted (as I often do if I'm talking to an expert), in order to improve their skills, I try to make it constructive and encouraging. No one likes to be put - down at any time, and I notice that it is sometimes uncalled for and out of jealousy.
Especially if someone is inexperienced, we need to encourage without too much "babying" or criticism in order to keep the interest alive and not intimidating.