I was a huge user of Arc until it became obvious it was no longer being supported. For me, it was all about the tab "model". The key things that made Arc work for me:

- Vertical tabs

- Drag & Drop between "sticky" and "ephemeral" sections

- Hiearchically nested folders

- Command palette with fuzzy match to open/recent tabs

- Meeting integration

There were some other nice features ("Smart" folders with GH PRs come to mind), but really 95% of my usage was due to the above. The tab model was so good it actually allowed me to replace "to do list" type tools with just folders of tabs.

For me, I think they had their target market all wrong. They built a bunch of design features nobody used and cared about (custom theming?), when the core pitch was very simple: People LIVE in their web browsers, but everyone just seems to accept that having 100 impossible-to-read chrome tabs open at the same time is somehow acceptable. There are so many browser "power users" out there in white collar roles that don't even realize a better world is possible. These people are not going to be downloading (and likely can't download) a sketchy sounding "firefox fork" named "Zen", but they will absolutely download a professional-looking Chrome fork with security guarantees that allows them to organize their work better.