arc which is useful, but it spits out about a page of output right now. It's also approximately chronological (most recently added features at the top) but should be more hierarchical. I think this could be made more useful by having arc which produce a summary and prompt you to dig deeper.
For example:
$ arc which COMMIT RANGE If you run `arc diff`, these changes will be sent for review: a b c For details, run: arc which commits REVISIONS The working copy is not associated with any revision. For details, run: arc which revisions REPOSITORY This working copy is associated with the Phabricator (rP) repository. For details, run: arc which repository