I believe a dislike button stifles discussion. I would rather my users post exactly why they give a negative rating like a dislike than just click some button. A like button is needed as it stops people on my site saying stuff like 'I agree ' etc which are low quality posts in my eyes.
I use post ratings add-on and have had it customized through further development. For example I have like, funny, off topic and inappropriate buttons. Off topic and inappropriate ratings are considered 'negative' ratings, rather than ppl disagreeing with a post, and only mods can see who gave those. Users cannot see who gave them negative ratings to stop tit for tat rating wars. Furthermore ever post deletion by a mod incurs a negative rating worth 10 pts. This way via the status graph we can see which users are problematic or suck up most of our time or the communities by posting inappropriately or by going off topic. Also we use a formula whereby if a post is mostly rated negatively by users its auto hidden from users. Stops trolls and spammers in their tracks. Great for community self moderation. Few other tweaks here and there too to assist my community.