I think this is a really important point. What Slack solves -- speaking in the general case, not specifically for FOSS projects -- is that it's an essentially o

I think this is a really important point. What Slack solves -- speaking in the general case, not specifically for FOSS projects -- is that it's an essentially out-of-the-box solution. Not just for non-technical users, although that's very important, but for all users. 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 Wait, that last one is out of date.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.