How it would work:
On my watchlist I have a given page watched, "Foo." An edit is made to Foo with the revision ID 123. If Judgment:Diff/123 doesn't exist, the entry is highlighted. This would be an opt-out feature; people who don't want this can simply click the "x" to get rid of the highlight option.
Why it matters:
Right now, watchlists only keep track of whether a given page has been viewed by that person. There is no effective way to divide patrolling labor between people. This would allow users to leave detailed feedback at the diff level and to share that feedback with other reviewers. This highlight feature would highlight the watchlist entries that have not received human feedback yet. This could lead to more efficient watchlist monitoring.