Bitbucket 4.8 contains two new features for better code review:
Commit Level Review of Pull-Requests
This provided per commit diff view and commenting. You can see changes performed in each commit by selecting a commit, or you can see changes of all commit combined. Viewing diff of each commit is especially useful to check changes performed for a previous comment within the same pull request. Previously it was more difficult because of combined view you had to search corresponding changes within all the other diff of previous commits. Now you can just select the corresponding commit, usually the latest one, and you can just see what has been changed since your last review.
Branch Based Default Reviewers
If you want you can assign default reviewers for every source and target branch combination. Additionally you can also specify how many users should approve the changes before the source branch is merged to target branch. This features makes determining reviewers a lot easier for implementing pull request based workflows especially for the teams which require a gate keeper review for different branches.