I couldn't for the life of me understand how many people I spoke to didn't see the problem that lack of threading and async catch-up posed to linear chat systems as Slack gained in popularity. to me it felt like a forgetful, prettified IRC with awkward sign-up.

Personally I find Zulip to have the nicest implementation of "topic zooming" as it still allows for a global linear view of all activity. Spectrum feels a bit more "channelized" and a bit white-space heavy but prettier.

Zulip's mobile client has come a long way in recent months too.

The hardest part though was trying to convince the community to migrate away from other social platforms. Meetup was (still is) driving me mad with their forums/discussions features which are woeful.