Comming from a gerrit background, I liked the fact that gerrit allowed you to mark a code review with a -1 or -2 (to block it). In stash you can approve a pull request to show that you agree with it but you can't mark the pull request as denied.
Declining a pull request is like abandonning a review in gerrit. I'd like to have a Deny button that you can toggle just so that it's easy to spot that a someone disaproves of your pull request.
Disaproving wouldn't block a pull request but it would be a great visual cue in the summary section to show that someone is protesting against your pull request (maybe put a little red mark in the avatar, akin to the green cross when someone approves).
