[freyr]

Lync? Oh, Skype for Business. I use Skype for Business on OS X (Oh wait, it's still called Lync on OS X, because they haven't bothered to update their client), and it's borderline unusable.

At the moment, Slack and Skype for Business have different feature sets. Skype for Business is geared towards voice and video conferencing, as you say. But now Slack is moving into Skype territory with video-conferencing services. Given their ability to deliver high-quality clients for multiple operating systems, they stand to make Lync (oops, Skype for Business) obsolete.