In my view; product design & build quality. Yammer were already doing most of what Slack does. Tiny Speck took that and UX'd the shit out of it until it was a joy to use.

Gotcha, it's the first corporate chat app I've used outside of Skype, so I guess I don't know how bad it was, ha. I've personally had trouble with how unintuitive Slack's UI can be sometimes, when it seems like they're just trying to find places for all their features. But it's gotten ridiculously better in their last redesign.