Whenever the activity view is opened, all changes are shown at one page. This view is quite great.
I used trac the last years for some projects and the activity view there had one advantage: Items, that were displayed for the first time, are marked as "new" in trac. Having this in redmine, too, this would be really great.
trac shows a green line on the left side of the activity view that marks new items.
Maybe you can adopt this?
I think redmine should remember what was the last time a registered or unregistered member (maybe via cookie) visited the activity view. everything that happend after the last visit should be marked as new.
Maybe you can integrate this into redmine.

Thank you for your work!