I don't think there's an example for headers in lists yet. I meant the rounded rect around the header looks more like something you find on the web.
Here's a quick mockup of what I'd consider more in line with Haiku's style:
For the time, I'd say down to the minute is precise enough.
(Hey, and mind the sentence-casing... )
I don't think it should become too colourful. Just using the standard named colours and tint a bit up or down depending on that colour (light/dark).
Not sure going multiline is best; it doubles the length of the list...
Maybe going for a column list view is nicer. I also like the alternate light/dark rows of the e.g. Repositories prefs.
With columns people could make the columns they're not that interested in narrower (maybe truncating the contents) or even hide them. After all, there's still the preview at the bottom. That preview might be put in a split or expando view, so people that are just fine with only the list can hide that.
One column could hold the notification caller's icon, so people could hide the respective column. That icon could be superimposed with a "mute" symbol when the notification came in when muted.
Your popup menu at the top can then hold "All", "Muted" and maybe other categories.
You could add a filter text control like in HaikuDepot, to have a full text search through all entries and only show the hits.
I'd also rather have the timeline reversed: most recent on top.