Most common code review comments I've encountered are simple enough to solve it in the same file, just by swapping reviewed code with fixed one.
That begs for a feature for accepting reviewers changes into authors code without a need to whole "open IDE at this code fragment, change, commit, push, ask for review again, track all comments" flow.
What I'm suggesting is "submit a change request to PR" functionality:
1. reviewer can create a diff snippet by selecting a number of lines that need to be changed and changing it
2. after submitting it will be displayed similarly to comment, but with additional buttons "deny change" and "accept change"
3. after clicking "accept change" code will be changed in the repository (using credentials of the reviewer)
4. accepting change will trigger another build (and all jenkins/tests/hooks configured for PR creation)
5. after clicking "deny change" snippet will turn gray and the only button left on it will be "unblock" (and short information about snippet being denied)
I think that this feature will increase the efficiency of teams working with pull requests policy and greatly increase the popularity of Bitbucket among developers.