One key example is the multiple teams feature: the ability to have multiple accounts logged in at the same time, and the ability to easily swipe between them on iOS or toggle on desktop is a huge advantage.
A quick look at HipChat indicates this still isn't possible [1].
Considering more and more people wear different startup / company / organisation hats not having this feature is a great way to limit participation / growth in a communications platform.
That Slack had this feature at the outset for me meant I've already spread use of Slack to multiple orgs and participate in all of them at the same time.
[1] https://help.hipchat.com/forums/138883-suggestions/suggestio...