It'd be great if we could "disapprove" of a pull request - as a contrast to approving. Its not enough to not approve until its ready. It'd be great to notify the author quickly that you don't like it. Much like the current Approve button the reviewer could change your status to approved later after updates, or comments, etc..
Declining the entire pull request is only great if you are the author and you have changed your mind and want to modify your code. Declining also hides the pull the request. A disapprove button for the reviewer would not hide the whole pull request - just signal the author that something needs to change.
Currently, we make a comment to show our "disapproval" (or whatever) status. A button would be great.