Also, please do try Zulip (https://zulip.com/) before jumping on Mattermost.
For me some of the real deal are:
- real threads inside each channel.
- each thread has independent notification settings, so you really are notified of only the things you care.
- threads can be marked as “done”.
There are other sweet details, like a small widget that manages date/time in any timezone (so no one has to do transformations or checks) and more.
And one of the best: you can use it on their installation or host your own! And it is open source (no Free/Libre, I believe, but at least OS).
I hope this is useful for y’all. 
PD: lately Mattermost has changed it’s presentation for military purposes. That is truly and profoundly disgusting to me.