At one point in the past Slack was new and shiny. It had much less features, but every one of them were crafted with UX in mind. People were raving about it, because for many it was the first time they could use something that had the nice properties of IRC chatrooms, in a nicely polished package, and it was usable in a work setting. It brought some fun back into the workplace where you previously had boring email inboxes.
I don't know exactly what changed. Maybe with feature bloat, the same care for UX was lost. Maybe the competition caught up and it doesn't stick out as that special anymore.