Nice idea. Is like in the facebook

But i don't know if in a Forum web is good. (?)

Hedgewars Tournament 2015

Amateur Artist for Hedgewars & Tournament Organizator

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Do not do unto others what you would not want done to yourself

Many newspaper webpages that allow comments has a "like button". And youtube has gone one further with like and dislike button for the clip and comments.
But dislike button isn´t widely used on pages with commentaries, probably because it is a bit negative and some people will press dislike just for the heck of it.