There something odd about the UX here.
I see this as moving to a more facebook style Posts+Replies for team channels, with a more prominent reply box.
However - currently I find our teams use quite a lot of channels like chatrooms, not like facebook - they don't see posts and reply in thread; they just add another message to the main channel like a chat conversation. They're going to be very annoyed when things happen "backwards".
While this move towards posts+replies isn't bad in itself, it means we are effectively losing the team chatroom for each team.
Having a flag at channel level to say whether it's a chat or a posting channel would be good.
Ultimately all I've ever wanted was a groupable list of "stuff" going on that I can keep my eye on or get alerted to - and get involved where needed.
My ideal is for the app not to be split by feature at the top, instead being grouped by team (and arbitrary groups I define). I want to see tasks, calendars, chats, post-channels, calls, activity, folders, collaborative files, emails, apps, etc relevant to that group ... under the group. I wouldn't mind nested subgroups too, but can live without ;)