When I open my team's phabricator install, the first question I'm usually looking to answer is, "What tasks & revisions am I not fully up-to-speed on?"
The activity feed and email/notifications help, but there's often too much noise.
What it could look like
My knee-jerk UI implementation would be to simply place a small icon next to "updated" objects... here I added a blue asterisk next to the task #:
Once I view an object and return to the workboard, the 'updated' icon would go away.
The knowledge that I'm absolutely caught up to everything my team is working on and discussing... it's "inbox zero" for phabricator.
In tandem with this feature, the comments/changes history on a Maniphest task could have a visual indicator highlighting the point in time you last viewed the object (i.e. "everything below here in the history is new:")