Feature-wise, pretty much nothing, it's just targeting a different set of users. Slack started as a solution for companies and pretty much stayed there, Discord started as a solution for gamers and expanded from there.
Slack is Discord for adults, but adults are never the ones who make or break social media. It's younger people who tend to be more invested in the platform they use. It's not a rational choice (nor was Facebook for my generation), but it got kickstarted out of necessity (as a substitute to the limited chat options within games) and right now, it's the convenience that drives it further. Why switch over when, unlike Facebook or Slack, everyone you know is already using Discord?
So basically what we need is Discord/Slack, but as an open protocol.
The entire concept of having a gigantic centralized social network on top of a gigantic decentralized communication platform (the internet) seems like a bit of an anti-pattern to me. Email, Usenet, and IRC seem to have never gotten the successors they deserved. Centralized, closed, and operates by a for-profit organization does not count.
As a side note I’m surprised there isn’t more mention of message boards here. Their peak seems to have been 2001-2010 and then their importance faded. One can discuss the shortcomings of phpBB and vBulletin at length, but their basic function seemed to have worked extremely well.
Slack is Discord for adults, but adults are never the ones who make or break social media. It's younger people who tend to be more invested in the platform they use. It's not a rational choice (nor was Facebook for my generation), but it got kickstarted out of necessity (as a substitute to the limited chat options within games) and right now, it's the convenience that drives it further. Why switch over when, unlike Facebook or Slack, everyone you know is already using Discord?