The moat is the integration points. The real differentiators for Slack are centralized history (better than IRC) and easy web endpoints for integration (better than pretty much anyone else). After a company invests significantly in a myriad of integration tools (both external and homegrown), they have little incentive to move. That's Slack's moat.

Still, they have no real presence in the consumer space and that makes them vulnerable to the first player that can really cater to both businesses and consumers. As it is, consumer vendors (MS/Skype, Facebook, Google) are fumbling in the biz space, but that will not last forever.

Edit: have a look at Slack 3rd-party apps for what i mean by integration. Because it's so easy to build them, there are tons.