Re: Its the integrations stoopid
"Slacks USP is the integrations. It basically makes the hipster devs never have to leave it. It essentially becomes their comms and notification dashboard. Once you have JIRA, Jive, SNow, Confluence, Webex, Quip and half a dozen others all configured they are in happy land."
Urm, while it may be true that Slack integration may be easier to set up in some cases, Teams is certainly no slouch for integration and indeed, it is pretty easy to set up your own integration via webhooks.
A quick check turned up existing integrations for JIRA and Webex, Confluence and several options for ServiceNow. All the usual suspects like Twitter, etc are also there.
Teams has the ability to either integrate messages into a discussion OR expose a web service as a Tab within the native Teams interface which is somewhat different to Slack.
Teams (at least the enterprise version) also uses Azure AD for single sign-on so SSO integration to thousands of 3rd-party services is trivial.