It would be nice if differential let one mark a comment thread as "Done". The code review page could show number of pending threads (aka threads not marked as "done"). And perhaps have a dialog to show all pending threads. The whole idea is to let both change author and reviewer focus on unresolved comment threads.
I find it very easy to leave comment threads missing an reply. There is no easy way to query all comment threads were not replied. One needs to go through each diff, and verify each thread, one by one.
The way I see this being implemented is by introducing a status filed for the comment thread (assuming comments are grouped in a thread). The idea is that each time the **reviewer** comments in the thread, the thread switches to **pending**. When **author** replies, it switches to **done**.
Sometimes, **reviewer** makes comments which don't really need a reply. The status could be a combo field, with **pending** pre-selected but reviewer could change it to **done** meaning that comment is just an FYI. Likewise, **author** could use **pending** status to signal **reviewer** the thread needs his/her attention.