The second thing it solves is management buyin. At my company we used Lync and I tried to introduce Hipchat, but nothing worked until the company President explicitly bought in to the idea of Slack. Not sure who or what convinced him but he made it clear that it's the tool he wants to use. Pronto, all 500 employees (baring some of us older guys), clicked their heels and marched.
IRC works great as technology and I love it. But Slack solves management issues and as a company I think that's their biggest advantage.
A lot of smart people who are old enough to be familiar with IRC see it differently. See comments from old HN thread:
https://news.ycombinator.com/item?id=10486541
Many many not use Slack but they certainly can see the value of it over IRC.
Make an free/opensource product that will do that for me with no configuration or administration effort and I'll gladly use that instead.
HN loves to hate it and I know I should treat it as unencrypted as the crypto might or might not be good but both mobile and desktop clients works nicely (actually IMO brilliantly) across Android, IPhone, Linux desktop and Windows desktop. Bonus points for bot API and channels.