What's the basis of 'rating' a discussion?

Basically when it comes to discussions I rate the ones I respond to because I wouldn't have responded if I didn't think it was a decent question, or topic starter. I then rate the responses I think are really good, but not all of them. I do rate those who respond to my topics so long as they stay in topic, as a kind of thank you for responding. So far I have not encountered it here on mylot, but if someone is totally out of line obnoxious or cruel, or so vulgar that I have to rate something, actually in the one instance I didn't rate it negatively I reported it. As the person was being crass in relation to the issue of children, the tone, and suggestion was abusive towards children, while the poster may have meant to go for a cheap laugh it fell horribly short, it was on another site though. So in general I will rate those topics I respond to, and to those who respond to my topics, and to responses in topics I may also respond to where I think the poster had something exceptional to say.

No i don't rate each n every discussion. Till now hardly i have done negative rating once but have done positive some 10-12 times. When i'am impressed or the response satisfies wht i need, i do positive rating.