First time I used it, it was a bit confusing, as I didn't really know what to expect. Is it email? Live chat? A forum?

Turns out, it's a bit of both, and trying to use it as any single one of them, will probably make it harder to use.

Basically, I treat it as a live chat where it's easier to participate in different topics at the same time, Slack on steroids where everything is a "Thread" basically.

Sounds similar to Flowdock model? I greatly preferred that and was so disappointed when Slack finally rolled out their version of threading. It felt almost useless in comparison.

Alas, Flowdock is dead now and I've learned to embrace Slack as the better remaining alternative over Teams or Google Chat.

That’s what the grandparent comment said, what specifically do you find wrong with the UI? I for one really enjoy it and the keyboard shortcuts.

I think it's better than slacks sidebar threaded reply. It also has better code blocks, less irritating pop up magic and no bots nagging you to fill in credentials all the time.