[ericclemmons]

The UX is top notch. Not only is the UI polished, but big details like mentioning someone and an invite link appearing is awesome. Or force-pinging someone on DnD because of an emergency.

Slack is seriously user-centric, all the way down to how they run their Twitter.

[flukus]

Have you used it on an under powered laptop? It's dog slow and gets exponentially worse the longer you have it open. Don't even thinking about scrolling back through the history.

[azinman2]

They didn't start with all that, but somehow amassed a lot of users anyway. I believe it's more about the personal networks of the original team, but that's just a guess.