The threading, which they sell as the core feature, is nice, and does make it more usable for "important" conversations than purely chronological chat. It also makes it easier to screen out conversations you don't care about.
But IMO the killer feature is the "all messages" view that merges chosen streams in more or less chronological order. It makes it much easier to keep up with what you care about asynchronously without throwing everything into a single stream. And therefore it is easier to put down, and to use notifications very selectively, which mitigates some of the major downsides of company chat.
I haven't used Slack recently either, but as I understand it its "all unreads" feature is much clunkier.
I haven't used a recent version, but I would definitely consider using it again.