The like/dislike post I think of reddit, which can get pretty mean-spirited (which is part of it's adorable-urchin quality). How would you/I feel if we got a -20 on a post that we thought was well-intentioned? Also can lead to mean-spirited voting, I dislike user X, and always down vote his/her posts regardless of goodness of a specific post. *shrug* always try to weigh potential good against bad.
How is it any different than a nasty reply or someone doesn't understand the context that you are replying in and gets upset?
If we currently had a dislike button then you could have disliked my suggestion and we'd know a little more about
the feeling of the collective forum without a survey or a hundred reply posts.
If you're going to post something on the internet then you're taking a chance of getting your feelings hurt. I'm a big boy and can handle it and that's what the ignore button is for.