edit: "All" they did was come along at the right time and make the interoperability easy. The prerequisites to have done it at all? Smartphones, WebRTC, and ElasticSearch.
I wonder if we need a new word for "interoperability in the time of SaaS" - the kind of where SaaSes talk to other SaaSes via locked down APIs, under absolute control of the vendor.
In order to do what Slack is doing, they have to be somewhat committed to open standards. They are just doing webhooks, like GitHub does. We tried specifying microformats, and we tried specifying webhooks, but the "loose RPC" model... Seems to work way better than XMPP server interoperability ever has.
I recommend this recent nested Twitter ("new crowd") thread amongst Stewart Butterfield and many other early web folk, about whether Slack is a web app: https://twitter.com/stewart/status/961704310613491712
Capturing email, files and love communication drives subscription revenue for E5 SKUs of 365, plus most companies will need Azure AD to meet their security and compliance requirements.
We ran the numbers at work... taking the blue pill and going all on Microsoft will 3x our lifetime value to them vs a standard E3 shop.