UX isn't just graphical design, it's also feeling responsive, fast, and accessible. Something that Slack, in my opinion, gets horribly wrong.
Honestly, I can't find fault with it. It's such a basic and uncomplicated piece of software, I'm not sure how they could get anything wrong.
A lot of discussion from this post from yesterday: https://news.ycombinator.com/item?id=20227115
Slack is certainly no email replacement, and nor will I ever use it for such but in my experience and for the purposes I use it for, it is far, far more featured than IRC, unless you're using some uber IRC client that I'm unaware of.
I'm genuinely trying to understand out how anyone would think IRC is better than Slack and I speak as someone who used IRC from the early 90s up until only a few years ago when the communities I'm part of upped sticks and moved over to Slack.
However, for me personally, a major attraction of IRC is that I can easily use it from within Emacs, and automatically have the same Emacs key bindings available that I also use for navigating and editing files.
With IRC, I can therefore quickly copy and paste text between chats and programs, search the history etc., all in the existing Emacs session, using the same key bindings, with direct access to built-in features such as autocompletion, dynamic abbreviation and spell checking, and — importantly! — without switching applications.
We all have our own ways of doing things and I personally prefer to have applications run independently so I like my Slack client and my IDE and terminal etc. etc. separate.
I've always found if one tool can do it all, it's generally a worse experience than dedicated software applications and doing a Cmd-TAB (I'm on a Mac) is just as simple as switching to another window in a single application
Personally: responsiveness. Slack's native app on linux is slow. Switching channels has a noticeable delay, switching Workspaces takes seconds if you haven't used the workspace for a while. Starting the program takes ~8s on my machine (three workspaces, i7 with SSDs and 32G RAM).
For most IRC clients or Telegram, startup times are about as fast as Slack changes a channel, and there are no noticable delays when switching channels/IMs etc.