I've used everything from IRC and ICQ through XMPP and HipChat to Slack, Discord, and Teams and let me share why I think Teams is so much more than all of its predecessors.
As someone who manages a large Active Directory domain and Office365 setup, Teams comes free. All I had to do was install the Teams client on users desktops and everything just worked instantly. Users logged in, installed it on their mobile devices, all secured by O365 2FA. I created an AD group for managers and all of them got the ability to send out Teams meeting requests via a local dial-in number for $4/month. When they schedule a meeting, the attendees can use Teams desktop app, mobile app, web app, or just dial-in via phone. Without this $4/month, users get everything except the dial-in phone number.
I created a few "Ask [Department]" channels in Teams and my users are using them in the appropriate way without over posting. Users message others in groups of 2-5 and create their own channels without asking IT. My users have conducted Teams webcasts for 40+ attendees, frequently video chat with groups of 8+, and have drastically reduced texting via their personal cellphone or using the old internal IM client. All of this without any training from IT.
So Teams is different because except for email, it has replaced text IM, SMS/MMS, forums, audio conferencing, video conferencing, and screen-sharing. It can also replace file-sharing and co-authoring but for business reasons I disabled that as much as I could. Also it automatically integrates with Outlook Calendar so users are alerted the same as before, just with a Join URL now. We were paying over $2k/mo for audio conferencing and now it's barely a few hundred dollars for a better, more stable product. Other than audio dial-in, Teams is effectively free for us because we have been on the O365 Enterprise plan for years now.
I last used Slack about 6 months ago and back then it had poor audio conferencing and no built-in video conferencing or screen-sharing. I was hoping after they bought Screenhero, at least screen-sharing would be native quickly but that was not my experience. Overall I like Slack, just that for enterprises already on O365, Teams is a no-cost no-brainer. Not even going to try setting up Slack.