Can't work on 2 different channels at the same time.
While working with planner (for instance), if a user, not related to the channel those plans belongs to, sends me a chat, notification, mention, I can't respond and continue with what I was doing before. I have to close the planner channel (for instance) and then open it again and look for what I was doing before.
Sean Ellis commented
All channels and tabs should have persistent state, and ideally be able to be "torn off" into separate windows so that they can be seen at the same time.