I was on the same boat given that I have used IRC for since the 90s. The one thing that Slack does solves is IT. It makes IRC "enterprisy" which means that IT can do account management, tie it with authentication etc..

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.