Users still consistently get hung up on understanding the behavior of arc diff. Some specific things we could improve:

• My experience is that arc which is helpful to users, but users often don't know about it. arc diff should provide a clear path to discovery.
• It might make sense to provide an abbreviated version of the arc which output in the arc diff output?
• There's some very old, crufty default-base-revision code. We should remove it and move completely to base rules, which have proven powerful enough to express all desired behaviors.
• The default behavior of arc diff should probably be to prompt / walk through?
• Users still consistently have different expectations of what "review" means. We might be able to surface some of this documentation more prominently.
• When the arc diff is updating a revision, we could attempt to introduce an arc:last rule which uses the base commit of the previous revision. This is complicated because we need the base commit to figure out the revision, but in the explicit --update case we know it and the general case we might be able to figure it out (e.g., by looking a few commits back in history?)