One thing I liked was the use of WikiCase. Just type the title of another page, and it makes it a link. No markup to learn. Very approachable. There are a number of major downsides, though, so I am glad most modern wikis use a markup-based system.
But the tiny learning curve made them so accessible. Back then you could introduce a wiki for your team and start churning out project documentation right in a meeting. Where the wikis back then didn't hold up was for long term documentation sites. If your wiki would only have a short shelf life it was an awesome collaboration tool.
Where did you feel wikis fell down in the long term? I would quite happily ditch Slack for something like a live collaborative Google Docs but simpler and with wiki style linking, if the tooling for search and discovery was good.
http://wiki.c2.com/?WikiCase
http://wiki.c2.com/?WikiNameDisadvantages
http://wiki.c2.com/?WikiNameAdvantages