Thanks to its distributed design and because no person owns it, it’s away from any kind of advertisement and corporate interference. After making an account, you retain the ownership of your personal data. It’s also better than Facebook for people who wish to hide their real identity as it allows pseudonyms. You can use hashtags, mentions, text formatting, etc.
A simple chat (slack style) it's enough. Just having the possibility to send messages either single user or team (#channel). But please don't forgive to give an useful appeal to mobile app (worst error of competitors is, have a good app but totally no mobile friendly!) i would expect to have a similar whatsapp or slack app for messaging within the slack mobile app!