IRC was one of the first things that got me excited about the Internet - I was studying Italian at the university, and realized that I could actually communicate, in real time with people on the other side of the planet, for free, back in 1993. I was utterly amazed by this. At first, I figured the people in the chat room (#italia) were BS'ing me and must just be Italians that lived in the US or something. A few years later, I managed to meet several of them in person when I moved to Italy for the first time.
Today, among others, I hang out on #startups on Freenode. It's not a very "serious" channel like some of the programming language ones are, but it's fun and interesting sometimes.
Slack has a lovely web client and both XMPP and IRC gateways -- meaning most everyone can use their favorite interface.
People who are unresponsive to email, Tweets and other means of communication can be very responsive on IRC.
More signal less noise, I quess.
I also really miss how in IRC channels are kinda the default way of communicating. I feel like Skype and others start from one-to-one communication and channels/rooms/... are more like a secondary feature.
If at work I need to ask some question, I do not care who answers to that and in many cases I do not know who has the answer. I do know what team or interest group might know the answer. With channel first kind of IM (IRC) I just join #folks-who-develop-the-server-im-sending-data-to and throw the question in the air. Eventually someone answers me.
With Skype, there is no such channel, because the mindset is different. So I need to browser the corporate intranet to figure out who might be the tech lead of the server I'm sending data to and contact that person. I chose the tech lead because he's the person who most probably knows the answer. Too bad that he is also the busiest one and I'm just making the situation worse.
Also, I really miss news groups. Why each web forum must reinvent the wheel and fail at it? :( I'm not that old, only 31. But boy where things better when I was younger. :D
I'm not aware of any platform that combines a reasonable API/protocol with a good web experience. I have some hopes for mailman3, is a bit sad that lampson appears dead, that edgewall killed their webmail frontend (I thought it might make a good basis for a proper web frontend to a proper mailinglist). And there is of course the D forums that as I understand it build on usenet news as the backend, and with some styling/templating might make a decent web forum.
With IRC, all new users are told (by their friens and pretty much all tutorials) to join a channel first. Also web clients tend to ask user to give an channel to join before they can do anything. For me and many others IRC is channels first. Commercial IM systems are one-to-one first.
There are obviously lots of additional little features (emoticons, inline images, etc) -- but at least with Slack, you can setup an IRC connector and just use your plain old IRC client.
We currently use OpenFire/Spark, which I like but my users find the "room" (called conference on OpenFire I think) hard to use.
Slack also uses room features.
Edit: I just saw that MetaCosm already said it.
It's sad that we can now, essentially, not do much more on the net than we did 20 years ago, but now it has ads and spies on you.
A few years ago, I tried to convince my colleagues (they're more designers than programmers) to use IRC for informal communication. Using IRC was (and still is) a no-brainer for me. My desktop PC runs all day, so I'm in IRC all day. But for them, joining IRC meant opening their laptops, starting their client and then pinging out 15 minutes later when their laptop went to standby.
When they were in IRC, they had this urge to really be there in IRC. I.e. monitoring it, reading everything, responding in seconds, excusing themselves when they had to go to the bathroom, and finally, finding excuses to close their IRC client again.
I have a totally different mode of operation with IRC, where it's OK to have the client open and not respond for hours or at all if you don't fell like it. I guess it's something you have to learn.
Installing BNCs solved some of the issues, but not all. Also, installing a BNC is not something your average designer-type nerd today wants to deal with.
So we ended up with KickOff for a while, but it never really worked reliably. We're now on Slack and quite happy, but I still want to try some of the various IRC "cloud" services; mainly those for self hosting.
I am not well aware of history of XMPP and IRC. I wonder if lack of interest in the organizations that control these protocols have lead to numerous versions of them that there are today. Slack has an IRC gateway and Facebook uses XMPP. Both of these protocols are widely used but have becomes too different from the core concept.
It's an amazing solution. So amazing, that I actually started commuting to the Quassel Android client, just because I want to give something back to this community.
Also, lack of addressability. IRC is primarily targeted towards channels while all the needs of current IM systems are targeted towards one-to-one messaging. Possible with IRC, but you have to jump through hoops.
For what it's worth, I agree that it's a shame that we are barely better off than we were 20 years ago, but I'm willing to accept that if it means widening the audience of the Internet for everyone. You shouldn't need a CS degree to surf the world's information.
Automatically connect to freenode / quakenet / other major networks, sometimes even intelligently enough to try to reserve the username or users contact info nickname.
Hit a join channel button, or just get prompted the first time to name a channel.
Ommit #, it works fine. It also gives you a picker of servers, defaulting to freenode in konversations case.
Join a channel and chat.
But there is no means to notify users this exists, or how it works, or how to get on it. There is no money to be made in getting everyone on IRC.
Interestingly, Twitch uses irc in its backend to power all of the realtime chat that accompanies its streams.
They offer a web client, which also stores channel history when you're away. And there's also an iOS client that will send you push notifications when someone mentions your name...
I'm not a big fan of terminal-based IRC clients. Most text on IRC is prose made by humans, meant to be read by humans. Terminals use fixed-width fonts. That's handy for structured stuff, but for prose is less information-dense and less readable.
Have you seen QuasselDroid? It's the Android client for Quassel, I'm working on a Material redesign for it right now ^_^
You should also try the quassel webproxy (a web interface like IRCCloud) and the quassel-search tool, which allows you to search through the whole logs of everything.
My transgression? I claimed that I used IRC in 1993. Which was impossible since, I was informed, IRC had only been around since 2000-something. :)
A channel mod informed me that, because Linux didn't use Apple's proprietary advanced power management protocols (which don't exist, by the way), Linux would overheat and utterly destroy my CPU.
After insisting that this was not my issue, and that what he was saying was ridiculous, he permabanned me. I looked him up online, thinking this was surely a joke, and it turns out this guy is a well-known weirdo with a pathological fear of open source software. It was very strange, and I have no idea how such a person got a mod position in a tech channel.
As others have noted, I think IRC is one of those things that will just keep chugging on, unaffected by trends.
It's been great for getting to know people all over the world. Relationships started on IRC have, among other things, given me my wife and my current job.
I fear that cannot be fixed in a backwards-compatible way.
I haven't used IRC in many years, but started using bitnet relay back when I was about 15 or 16 and then moved on to irc (mostly +Amiga!/#Amiga!, +hack/#hack, used it back when all the channel numbers were just integers as well on various channels) soon after Jarkko had it up.
I remember when having 50 people logged into the whole of irc (long before all the splits into EFNet and then all the others) was a really busy night.
Ended up meeting a lot of cool people through irc and pissing off a bunch more as I was a bit of a troll back then (well, more of a troll than I am now), pretty stereotypical geek kid with more intelligence than wisdom.
I miss the 1990s with IRC them days were unreal and so much fun. Today I think of it as a tool to get help and to help others and that is about it. Seems like channels get smaller every year except a few (Arch Linux and Ubuntu.
Here's a small sample from the FreeNode IRC network, from right now:
| Users | Channel |
| 1751 | #ubuntu |
| 1620 | #debian |
| 1590 | #archlinux |
| 1488 | #haskell |
| 1477 | #python |
| 1109 | #gentoo |
| 987 | #vim |
| 877 | #ruby |
| 815 | #emacs |
| 652 | #perl |
| 452 | #java |
| 417 | #lisp |
| 275 | #startups | 
| 190 | #scheme |
 - #startups - the HN channel!
 - http://searchirc.com/networks
Still, though, that model seems unsustainable: fewer and fewer people are joining IRC channels or getting into what used to be a "scene." I think there will always be die-hards on IRC, but there's no longer the coolness that there was fifteen years ago.
I guess the point of my comment is to say that maybe IRC use has changed, but it still is an effective and active communication tool that many people rely on daily, among which some have only known IRC the way I described it: as a communication tool for group of friends (maybe a bit like social networks).
The ban evading trolls and constant net splits caused by frequent DDoS attacks were a problem. ISPs stopped hosting IRC servers because as soon as a server would come up, it would be a huge DDoS target. Then the overbearing fear of "illegal material" flowing across the pipes was a problem for others.
But I'm glad that it's still around. One channel in particular I live in has been around for 15+ years now. It's like a digital version of Cheers, without avatars.
Such a national activity has to have good communication tools, and IRC can't be beat.
There are a few channels I know that are mostly active over the workweek in US pacific time. Others that wake up with the EU, with some extra activity on weekends.
But yes, some channels really are slow and mostly wake up only on specific actions (eg. joining, asking for a question, people answer, everybody gets back to doing their work) - essentially support channels.