When I'm doing a code review that includes a lot of back and forth on several files, often one file will be fairly easy to review and another will be more difficult. For an easy file, I only need to look at it closely once, and then I never need to look at that file again unless the code author has changed part of it. However, I might need to look at a hard file 5 times if there's a lot of back and forth on it with either code changes or comments.
The problem: when I'm doing the second through the fifth pass at the hard file, Differential shows me the easy files again even though I don't need to look at them again since nothing was changed and there are no comments on it. If I could mark a file as "done" or "reviewed" or something at a particular point in time in Diffusion, then it would know that it didn't need to show me the file again unless there was a new comment or a new change in the file.
This wouldn't be a problem for me if I compared, for instance, Diff 4 to Diff 5 rather than Base to Diff 5. However, with complicated code reviews, I usually diff against the base rather than intermediate diffs because intermediate diffs can make the review more complicated (eg, if someone makes a change and then reverts it, diffing against the base would show no change, but diffing against the intermediate would make it look like there was a change).