I do not give a shit what Slack's buttons look like. I can't do that with IRC. Even if we're all standardized on a modern OSX IRC client, we're still going to end up "sharing" URLs.
Of course, what you mentioned is only one of many things that Just Work in Slack and are at best a pain on IRC, but still.
I do care what Slack looks like though - the chat window has so much wasted whitespace that it doesn't show a lot of comms; add in a space-guzzling integration or two and it gets really hard to follow a conversation. Hipchat uses space a lot more efficiently.
At any rate, on Slack, I once drag-and-dropped a text file with the output of a shell script that a colleague wanted to see. He started telling me how it was strange that the output seemed to cut off right in the middle of a line. lol, just kidding, Slack was just cutting it off like that, since Slack apparently can't show more than the first few K of a text file for some reason SO WHY IS IT SHOWING IT AT ALL ARGH
Not every organization needs this kind of workflow though, so the best available tool depends on what your organization has to integrate. (Disclosure: I've never tried HipChat.)