A common request, and challenge with the current UI, is being able to reply to comments in the diff viewer. The popularity of code review services (like GitHub's) where all review and discussion is performed within the diff viewer has increased the number of requests for this, and it's just generally the most common model out there.
When Review Board was first created, there wasn't really a model in place. We instead have a separation of the discussion area (reviews on the review request page) and reviewable assets (diffs, file attachments), and while you can see discussions by hovering over or clicking comments in the reviewable assets, you can't really reply there. Lots of people have been tripped up by this, and this project aims to solve it.
The new Unified Review Banner solves the problem of publishing replies in a consistent way, which takes care of the really hard part. The remaining work has to do with creating a better UI for performing replies.
For more information, see the design document:
Replies in the Diff Viewer: Design
Put down any notes you have as you develop this, for future reference.