Threaded discussion support is basically the thing that's been holding Slack back from complete world domination, and for some reason they're taking forever to do it:
https://mobile.twitter.com/slackhq/status/578575540594020353
https://mobile.twitter.com/slackhq/status/578575540594020353
The same goes for Gitter, fwiw. They've promised threading too a while ago: https://github.com/gitterHQ/gitter/issues/741