Looking into a public channel you are not in, a thread now opens to read, with Join in place of its message box, and a reaction is yours to add. Pinning, starting a Link, voting in a poll, commenting on a list item, adding a canvas and running an app’s command there are the channel’s members’ to do: they are no longer offered, and the bar reads Join to post and hear about what happens here. You can still join a Link that is already live, and add a colleague.
New message offers only the channels you are in and can post in. The sidebar’s pill for what is below the fold now counts every unread conversation there, not only mentions. A line saying somebody pinned a message, set the topic or added a person names them as it arrives. A file sent with a message is shown at once, to you and to everybody reading. And Pinned shows 25 at a time.