I use slack at work. Apart from a few special clipboard paste types (like images and other file type sharing), the app seems to offer almost purely only written communication. Integration to other systems might be its strength (via its app directory).

By looking at "curse vs discord" feature list at the bottom of https://www.curse.com/, both curse and discord seem to have much more to offer than slack.

I had not heard of rocket.chat. By quickly looking at its features, it seems pretty similar to slack.