"Constructive criticism, of course, is a terrific tool. If a critic tells you, "I don't like it" or "This is disappointing," he's done no good at all. In fact, quite the opposite is true. He's used his power to injure without giving you any information to help you do better next time. Worse, he hasn't given those listening any data with which to make a thoughtful decision on their own. Not only that, but by refusing to reveal the basis for his criticism, he's being a coward, because there's no way to challenge his opinion."
Seth Godin, Tribes p.47