I just launched Chat in my community. And I thought that the “chat default channel id” setting auto-joined people to that channel. It doesn’t though and I had a moment of terror when I realized that wasn’t happening 
I’d love to see even a simple version of this that has a site-wide “default channel”.
Edit: Conversely, it would be helpful to be able to see how many people have joined a specific channel, so I can understand what the reach of posting there is.