When is a discussion considered "quality"?

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We can not overstate the importance of a well thought of topic for discussion. But how do we define and classify them considering a wide range of users. If the subject they have thought of doesn't seem to hit others as quality ones. Who will judge? To others, some may not be quality ones but they do get responses. Does this mean it is really difficult to define quality discussion? I have read topics like How many times do you have s*x? or what is your favorite color? Personally it doesn't strike me as quality one but if the one who posted it think it is, the subject is important to him, and that he could not think of other subjects, who am I to judge? Any thoughts on this?