[fensterblick]

If they can do this right, they will get a lot more customers.

Right now we use Lync/Skype for communication, which I loathe almost as much as Lotus Notes. This sentiment is shared by many. Slack usage has organically grown (as a reaction to the failings of Lync) and I can envision Slack displacing Lync in many companies.

[SinomaSo]

This. I think Slack is going a new territory with voice/video. It will compete now with MS Lync and Cisco Spark. Both are eating market shares in collaboration and Slack will be affected. The two giants are investing a lot of money in their cloud based collaboration tools and might want to buy slack in the future, if it becomes a real threat.