The big thing is that they let you use it for free, get hooked, and then when your team is all in on using it, paying the $60/user per year is worth the upgrade. They made their base product good enough that you don't need the full version... until you do.

There were interviews from last year where he talked about slowly converting teams from free to paid.

I use slack for a side project with 15 or so people involved and while we're using the free version, it is on the list of things to pay for once we have some sort of revenue.