I feel like a dislike button would be difficult to implement for two reasons:
Reason One, it can get kinda toxic, which is shown best by Reddit.
Reason Two, following the Reddit line of logic, this site has a level system and Likes seem to tie into that, so how would Dislikes tie into that? Does an unpopular but well thought out opinion get punished, or is the Dislike button just an aesthetic vent for your feelings of someone else's opinion?
Finally, I don't believe you should encourage the idea of disliking someone's opinion, in general. I feel that kind of thinking causes debates to turn into arguments, where you're just trying to "win" or "show up" the person you're talking to.