Too much white space in UI and not enough message density
There is far too much graphical 'white space' between messages in teams. It is not serving any value and is wasteful. For example move the 'Reply' button to the right of the message rather than below, the Reply button being below every message wastes valuable vertical space. On top of this there is a 'gap' or border and moat between each message. Compare to mIRC, Slack or other chat clients one can review much more messages and read more seamlessly without having to scroll as often. In other words, make it more chat client'y and less email client'y.
Sean Ellis commented
This is a very popular request - the main thread is here https://microsoftteams.uservoice.com/forums/555103-public/suggestions/17408641-please-create-a-professional-compact-chat-layout and already has over 4000 votes. Microsoft have already made tiny steps in the right direction, so please add your voice and help us to get a truly compact UI mode!