In a word, quality.

In a few more words, the product appears to have been designed with user needs in focus more than the others, and the end product is closer to solving them. The article reveals some of the ways they've achieved that. Furthermore, it goes beyond simply meeting user needs into a superlative user experience that also exceeds expectations. It's not too complicated: a big part of their success is that they have created a better product in the eyes of users.

Right from the 'Atlassian' in the very top left corner, Hipchat is completely and utterly devoid of personality.

In my opinion the big difference was a usable free tier, and the recent move to easily support logging in to multiple teams at once. Also general quality.

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.