1) beautiful design (like the rest of arc, also, unrelated, love how they solved the URL for the side-tabs - preserves all vertical screen real estate)
2) expansive commandset
see here: https://browserinc.notion.site/Command-Bar-T-Actions-e18b0069b1db45a99db1fc07d9ba4c41
especially
1) managing tabs with fuzzy search
2) managing history
here you can see how with one word written I query tabs, history and start a search simultaneously. Massive reduction in cognitive load because whenever I want to do anything I just press cmd+t, then start thinking about what I want to do and start typing until only remaining what I wanted to do - this is a lot faster than having first to decide what I want to do, then click the respective shortcut (e.g. having 3 shortcuts for command, history and tabs)
3) managing extensions (e.g. to turn off ublock for a site, I just cmd+t, then type ub<cr>
and voila I can turn it off
4) split views & managing them (again additional feature worth implementing imho, though separate request)
5) preferences