Directory tree and grouping
The file matrix now displays files in a tree structure rather than as a flat list. I think this is easier to read and saves considerable horizontal space, which can be a problem when you start accumulating a lot of revisions. Files are always listed before subdirectories, and you can hover over a directory to show its full path in case it's deeply nested and you lost track.
Another brand new feature is that you can now group files! By adding a bit of code to your custom review completion condition you can segregate some files into their own groups, to be listed separately in the file matrix. You'll then be able to collapse them and mark them as reviewed with one click. (The full discussion of the feature gives some more usage ideas.)