I've used and help maintain Lync and OCS, its predecessor. Both were significantly more bloated, difficult to navigate and laggy than Slack is. Additionally, you need SO MUCH infrastructure to support a proper Lync deployment, especially if you want video, voice and PBX. (Yes, Lync comes with its own PBX.)

Lync 2013 was even worse! The message window was laggy in typing words out of the box and every UI animation was choppy. This was on a really beefy Dell (I forgot the specs). I was stuck in this awkward position where I had to defend the implementation even though (a) many people hated it (myself included), and (b) I actually used Pidgin because it was so much lighter (even if user discovery and lookup wasn't as great).

Fuck Lync. I love Slack. I can see why Microsoft attempted to acquire them, and I am super glad that they didn't.