I can express disagreement with words. If someone dislikes what you say, they ought to be able to tell you why. I can imagine this being used mischievously, with someone disliking whatever a particular user posts, because of dislike for the user, rather than the content of the post.
I also think a dislike button would skew things. I was on another (now defunct) site, relatetothat.com, which had a "thumbs down" as well as a "thumbs up" button. The former was almost never used. I think the reason is that that site was so heavily moderated, that the only people who used it tended to be very polite folks. Since SW does not have that kind of self-selecting filter on its membership, I can see such a feature turning ugly.
Third, there are some people here who are emotionally vulnerable. We have read stories in the news of teen suicides over negative social media feedback. I see no reason to introduce extra negativity here.