In my opinion Slack does something that is often very undervalues and easily overlooked, it is like the opposite of a death my a thousand cuts - superiority by a thousand minor improvements.
When taken individually or even in handfuls none of the features are that impressive, but cumulatively it creates a much better product. This was very apparent to me at least when I switched from HipChat to Slack. Just reading features or looking at screenshots they are nearly identical, yet with actual use Slack seems like a significant step up.
So yes IRC is to Slack, as the Ford Model T is to a Mercedes S-Class.