The thumbs up/thumbs down icon is already used for voting on an issue and should not be used for follow/unfollow. It is particularly confusing after creating an issue, users cannot vote up their own issues (the vote button is not there), but the thumbs up icon from the follow project command may suggest that they can.

Submitted by Gabriël Konat on 18 January 2013 at 11:27

Changed into empty star (follow) and star (unfollow; or you are following this project). In github.