I like that Mattermost treads Channels as Groupchats, and you can reply to a Chatmessage or just add to the Conversation where in Teams you have Conversations where you can reply to or start a new one, this leads to people replying via new conversation or replying while it's a new conversation.

I also dislike the notion of Teams with Subchannels in Teams and the hard cut between Chat and Teams - I like the way Mattermost does it more: a <hr> Element between Channels and Chats.