-1 or the thumbs down reaction, I think is a mistake. They aren't usually that constructive, because most of the time they are used as retaliation against a specific user, instead of constructive criticism. If someone downvotes you, you tend to downvote them. At the least, it should be a privilege to down vote, like SO and HN do. http://stackoverflow.com/help/privileges/vote-down
I think that the -1 button definitely has a place on Github. I use Github a lot for discussing future feature development, and in that context it is really valuable to see how many people think a feature is useful or not. Github isn't primarily a social network. In a professional context it is essential that you can also express disagreement, or that you think something is a bad idea.
You know, "+1" usually has only one meaning:
- "I agree with whatever you're saying".
"-1" has however two separate meanings: - "I disagree with whatever you're saying because I believe that blah-blah-blah (reasons here)"
- "I disagree with whatever you're saying because fuck you that's why"
And "-1" button doesn't distinguish between those two cases.