Add Reactions to Pull Requests, Issues, and Comments
-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