The only other Slack alternative I found was actually better than Slack and not just a "sidegrade with different pricing and less integrations" was Flowdock - unfortunatley, now defunct.

It was all down to how Flowdock implemented threads. They did it better and long before Slack ever did; Slack's threads frankly kinda suck.

I've heard Zulip threads work really well but I've never had the chance to use it with a team that was willing to try something else.