discord is slack on steroids. also no weird limitations on big groups, and no per-member fee for bigger group.
Also, compared to slack, the discord team is much more approachable, is more open to bigger groups, and also more receptive to suggestions for improvements and additions.
We will soon even have webcam and other hangout feats.
we (the biggest ue4 group) had to move from slack to discord because we where hitting limitations on chat logs and more, and the lead admin crew would have had to pay a sum with more than 4 zero’s just to get proper file support back.
So we moved to discord, and they are happy to have us.
Our link is still funny though: http://join.unrealslackers.org/ (yes… thats a discord link)