Really? As someone who's been using Discord since 2015 the interface is by far the worst aspect of it in my opinion. The desktop client has a minimum width, the channel list is not resizable, Ctrl+K for some reason doesn't prioritize channels in the current server, no tabs, you now need 2 clicks to reach your mentions instead of 1, if you don't have access / don't want to use a scrollwheel the server list is literally unusable (and even then, it feel awful reaching for the last servers), the client lags insanely hard if you're in a voice chat (Linux 5.8.14 with RX 5700 XT), virtually no accessibility features (besides TTS, which admittedly was there from the start)...

These are the UI complaints I have that I can list from memory. There's a bunch more regarding to how the service is operated (especially support), but it's overall not really a good experience for me. Definitely better than Skype, though that's a very low bar.

Edit: Oh, and no threads. That's Slack's best feature by far.