Main thing that Slack solves as far as I'm concerned is a great and uniform client on every platform.
Don't get me wrong, I love irc. This is where I started learning everything I know about computer.
I tried many times to get various team on it and it always failed. I think it's mainly because the learning curve is too big for non-technical/busy people, and that there's no good and uniform clients on all platform.
Both the linked article and the various suggestions in comments give all sorts of IRC-based alternatives to what Slack does, but they're all things that, let's be honest, require a lot more work to set up and are often far more fiddly to use. "Oh, this is easy, just run daemon X for this functionality and daemon Y for this functionality, and set them all up using stuff you can get here, here and here. Wait, that last one is out of date. Try using this gist instead. Okay, now, you can get that client-side feature you want if you switch to a different IRC client, use the following .config file, and look for the following plugins, although you'll probably want to change the defaults..."
Compare this to Slack, which for most users is: sign up, click here, and in Jeff Goldblum's words, "There is no step 3." From a practical standpoint you have to install zero new pieces of software (assuming you already have a browser), you have to edit zero fiddly text files, you need to install zero Perl scripts. Once your IRC is set up it seems "easy," but it's possible to spend an inordinate amount of time finding the answers not to questions like "how do I get snippet previews of links to show up in the client like it automatically does in Slack" (the answer is probably that you do not), but questions like "how do I get all the nicknames to show up in different colors?"
For certain FOSS projects, this may be less of an issue because the team may be more experienced in setting up frankly fiddly Unix software, and I think having an IRC channel is a good idea. It may be all you need; what Slack gives you above IRC is generally "nice to haves" rather than "must haves." But the FOSS crowd, to make a very rough generalization, often tends to deprioritize "nice to haves" to a point where they miss, well, just how nice it really is to have some of them.
This whole discussion is similar to that Ogg vs MP3 file nonsense on Wikipedia; the victory of philosophical purity over practicality and usability. I still can't just click a Wikipedia sound file on my iPhone and have it just work. Victory for ideological purity big fail for the 98% of the world that doesn't give a s--t about file formats or Jimmy Wales's religion.
I sort of get why people like Slack. It's very low effort. But that's the only advantage I see.
That anyone find it hard to use is flabbergasting to me as it is a matter of opening a client, entering a nick and selecting server. With the web IRC clients it's even easier I guess.
I still hang out in several of the channels from my early days :)
If you tell people something is hard to use, they'll believe you. If you tell them how to use something, they'll use it...
With slack, persistence comes for free. The only thing I've seen that comes close is irccloud, which is in my opinion an excellent competitor to slack. In the end my organisation went with slack because it's nice to have everything grouped and billed to the organisation, as well as being even simpler for end users to join.
Really? Does using Slack not require opening a client application and logging in, like just about every other realtime messaging platform?
I know many "non-technical" people who use IRC.
I love IRC and grew up with it. But don't compare it to Slack for technical knowledge requirement.
In my firm at least, Slack is an incredible tool for our PMs and owner, and it also works well and covers all use cases for the consultants.
Use SSL + SASL auth and it's not any less secure than logging in to most websites.