When a friend asks your opinion about their labor of love, how do you tell them politely without insulting, or ticking them off?Excellent question and one that requires knowing your friend and having the emotional intelligence to find a way to express that in a positive manner that he/she will respond to in a positive manner. In order to do that, I simply ask my wife ;-) Women, in general, are often better at that kind of task (as long as it's not their husband...rim shot!) No really, they are. After 40 years, some of her knowledge and skill in that area are beginning to rub off on me.
There is a general trend here that I see. Some people (like me) are what I call gestalt listeners. They don't focus on the individual elements but rather the overall result in terms of the way that they respond to it. I know I can tell you which (insert component or system here) I enjoy more, but it's often hard for me to nail it down as to why. Some people are very analytical listeners or perhaps what I would call splitters (whereas I would consider myself a lumper). They can easily tease out the individual elements that make them prefer one sound over another. Taken further, they may not be able to enjoy something if one individual element to which they are sensitive is not exactly to their liking.
Whatever floats your boat. And maybe that is the way to approach the question of making constructive comments to a friend. Ask them what they particularly like about component X or system Y, agree w that, and then discuss the things that are important to you that you might find a little lacking.